Meaning: 'Sounds' refers to the way something appears or is perceived, often used to indicate an impression rather than a fact.

Wrong options:

• shows: The verb 'shows' implies a demonstration or evidence, which does not fit the context of perception as effectively as 'sounds'.
• evaluates: 'Evaluates' suggests a judgment or assessment, which is not appropriate here since the sentence discusses perception rather than analysis.
• serves: 'Serves' implies a function or purpose, which does not align with the intended meaning of how the situation is perceived.

Correct answer: acts

Explanation: The verb 'acts' is appropriate here as it indicates the role of carbon dioxide in the context of fertilization. It fits grammatically and contextually, as it describes how carbon dioxide functions in relation to plants.

Meaning: 'Acts' means to perform a function or role, in this case, referring to carbon dioxide functioning as a fertilizer for plants.

Wrong options:

• poses: 'Poses' suggests presenting a challenge or threat, which does not fit the context of carbon dioxide's role as a fertilizer.
• adds: 'Adds' implies an increase or contribution, but does not convey the active role that carbon dioxide plays in fertilization as effectively as 'acts'.
• pretends: 'Pretends' indicates a false appearance, which is irrelevant in this context where the role of carbon dioxide is factual.

Correct answer: account

Explanation: The verb 'account' is suitable as it indicates that agriculture and tree planting are responsible for a significant portion of greening. It fits grammatically and contextually, as it describes the contribution of these activities to the overall phenomenon.

Meaning: 'Account' means to be responsible for or to explain a certain amount or proportion of something.

Wrong options:

• charge: 'Charge' implies a cost or accusation, which does not fit the context of explaining contributions to greening.
• credit: 'Credit' suggests giving recognition, but it does not convey the sense of responsibility as clearly as 'account'.
• blame: 'Blame' carries a negative connotation of fault, which is inappropriate in this context where the focus is on contribution rather than fault.

Correct answer: Therefore

Explanation: The word 'Therefore' serves as a discourse marker that indicates a conclusion drawn from the preceding statements. It fits logically and contextually, linking the cause of global greening to its implications.

Meaning: 'Therefore' means as a result or for that reason, indicating a logical consequence of the previous statements.

Wrong options:

• Because: 'Because' introduces a reason rather than a conclusion, which does not fit the intended meaning of the sentence.
• In contrast: 'In contrast' suggests a comparison or opposition, which is not appropriate here as the sentence is drawing a conclusion rather than contrasting ideas.
• Nevertheless: 'Nevertheless' implies a concession, which does not align with the logical progression intended in the sentence.

Correct answer: scale

Explanation: The noun 'scale' is appropriate as it refers to the extent or level of something, fitting grammatically and contextually in describing the magnitude of human impact on the environment.

Meaning: 'Scale' refers to the size or extent of something, in this case, indicating the global level of human alteration of the environment.

Wrong options:

• effect: 'Effect' refers to a result or outcome, which does not fit the context of describing the extent of alteration.
• place: 'Place' refers to a location, which is irrelevant in the context of discussing the magnitude of environmental changes.
• view: 'View' implies a perspective or opinion, which does not align with the intended meaning of the extent of human impact.