Meaning: 'Ubiquitous' means existing or being everywhere at the same time; it emphasizes that language is a fundamental aspect of human life.

Wrong options:

• commercial: 'Commercial' relates to business and trade, which does not fit the context of language being a central topic in psychology.
• exploding: 'Exploding' suggests rapid growth or increase, which does not accurately describe the nature of language use in this context.
• equivalent: 'Equivalent' implies equality or sameness, which does not convey the idea of language being widespread or common.

Correct answer: explicitly

Explanation: The word 'explicitly' is appropriate here as it indicates that infants learn language in a clear and direct manner, which aligns with the context of the sentence discussing how language is acquired without formal teaching.

Meaning: 'Explicitly' means clearly and in detail, leaving no room for confusion or doubt, which is relevant to how infants learn language.

Wrong options:

• illicitly: 'Illicitly' means in a way that is forbidden by law, rules, or custom, which does not fit the context of language learning.
• tacitly: 'Tacitly' means understood or implied without being stated, which contradicts the idea of learning language explicitly.
• licitly: 'Licitly' means legally or permitted, but it does not convey the direct manner of learning that 'explicitly' does.

Correct answer: input

Explanation: The term 'input' is suitable as it refers to the information or stimuli received from adults, which is necessary for language development. The context indicates that minimal input was provided, leading to spontaneous language creation.

Meaning: 'Input' refers to the information or data received, particularly in the context of learning or communication.

Wrong options:

• application: 'Application' refers to the act of putting something into operation, which does not fit the context of language development.
• seperation: 'Seperation' implies a division or disconnection, which is not relevant to the idea of language acquisition.
• data: 'Data' refers to facts and statistics collected for reference or analysis, which does not specifically relate to the concept of language input.

Correct answer: gestures

Explanation: The word 'gestures' fits well as it describes the physical movements used by the children to communicate, aligning with the context of them developing their own sign language.

Meaning: 'Gestures' are movements of the body, especially the hands, that express ideas or feelings, which is relevant to how the children communicated.

Wrong options:

• fixtures: 'Fixtures' refers to items that are fixed in place, which does not relate to the act of communication.
• ruptures: 'Ruptures' implies a break or disruption, which does not fit the context of communication through physical movements.
• structures: 'Structures' refers to arrangements or frameworks, which does not accurately describe the communicative actions of the children.