Meaning: 'Responsible' means having a duty to deal with something or having control over something.

Wrong options:

• depending: 'Depending' is a gerund that suggests a conditional relationship, which does not fit the context of attributing causation to the viruses.
• weaken: 'Weaken' is a verb that does not describe the role of the viruses but rather an action that can be taken against them.
• transformation: 'Transformation' is a noun that refers to a change, which does not fit as it does not describe the viruses' role in the disease.
• harmless: 'Harmless' is an adjective that describes the effect of the substance on the viruses, not their responsibility for the disease.
• remarkable: 'Remarkable' is an adjective that describes something extraordinary, which does not fit the context of causation.
• governing: 'Governing' is a gerund that implies control or authority, which does not accurately describe the viruses' role.

Correct answer: deadly

Explanation: The term 'deadly' is an adjective that describes the severity of the disease, fitting grammatically and contextually as it emphasizes the seriousness of the hand, foot, and mouth disease.

Meaning: 'Deadly' means causing or capable of causing death; lethal.

Wrong options:

• depending: 'Depending' does not fit as it does not describe the nature of the disease and is not an appropriate adjective in this context.
• responsible: 'Responsible' does not describe the severity of the disease but rather the role of the viruses, making it contextually inappropriate here.
• weaken: 'Weaken' is a verb that does not describe the disease itself and is not suitable as an adjective.
• transformation: 'Transformation' is a noun that does not describe the nature of the disease and is not appropriate in this context.
• harmless: 'Harmless' is an adjective that indicates no danger, which contradicts the context of describing a serious disease.
• remarkable: 'Remarkable' is an adjective that indicates something extraordinary, which does not fit the context of describing the severity of the disease.

Correct answer: remarkable

Explanation: The word 'remarkable' is an adjective that describes the breakthrough, fitting grammatically and contextually as it highlights the significance of the scientists' discovery.

Meaning: 'Remarkable' means worthy of attention; extraordinary.

Wrong options:

• depending: 'Depending' does not fit as it is a gerund and does not describe the nature of the breakthrough.
• responsible: 'Responsible' does not describe the breakthrough but rather the role of the viruses, making it contextually inappropriate here.
• weaken: 'Weaken' is a verb that does not describe the breakthrough itself and is not suitable as an adjective.
• transformation: 'Transformation' is a noun that does not describe the nature of the breakthrough and is not appropriate in this context.
• harmless: 'Harmless' describes the effect of the substance on the viruses, not the breakthrough itself, making it contextually inappropriate.
• deadly: 'Deadly' describes the disease, not the breakthrough, making it unsuitable in this context.

Correct answer: weaken

Explanation: The verb 'weaken' fits grammatically as it describes the action that the discovered substance can perform on the viruses, aligning with the context of neutralizing their effects.

Meaning: 'Weaken' means to make or become less strong or powerful.

Wrong options:

• depending: 'Depending' does not fit as it is a gerund and does not describe an action that can be taken against the viruses.
• responsible: 'Responsible' does not describe an action but rather a role, making it contextually inappropriate here.
• transformation: 'Transformation' is a noun that does not describe an action and is not suitable in this context.
• harmless: 'Harmless' describes the effect of the viruses after being weakened, not an action that can be taken against them.
• remarkable: 'Remarkable' describes the breakthrough, not an action that can be taken against the viruses, making it contextually inappropriate.
• deadly: 'Deadly' describes the disease, not an action that can be taken against the viruses, making it unsuitable in this context.

Correct answer: harmless

Explanation: The word 'harmless' is an adjective that describes the state of the viruses after being weakened, fitting grammatically and contextually as it indicates that they pose no threat to humans.

Meaning: 'Harmless' means not able or likely to cause harm; safe.

Wrong options:

• depending: 'Depending' does not fit as it is a gerund and does not describe the state of the viruses after treatment.
• responsible: 'Responsible' does not describe the state of the viruses but rather their role, making it contextually inappropriate here.
• weaken: 'Weaken' is a verb that describes an action, not a state, making it unsuitable in this context.
• transformation: 'Transformation' is a noun that does not describe the state of the viruses and is not appropriate in this context.
• remarkable: 'Remarkable' describes the breakthrough, not the state of the viruses, making it contextually inappropriate.
• deadly: 'Deadly' describes the disease, not the state of the viruses after treatment, making it unsuitable in this context.