Meaning: 'Shape' means to influence or determine the form or character of something, indicating how plants affect the environment.

Wrong options:

• affect: While 'affect' is a verb that means to have an impact on something, it is less specific than 'shape' in this context, which implies a more active role in forming the environment.
• model: The verb 'model' suggests creating a representation or example, which does not fit the context of how plants influence the environment directly.
• form: The verb 'form' implies creating or constituting something, but it lacks the nuance of influence that 'shape' conveys in this context.

Correct answer: degradation

Explanation: The term 'degradation' is the correct choice as it refers to the deterioration of the environment, which aligns perfectly with the challenges mentioned in the context of food, energy, and environmental issues.

Meaning: 'Degradation' means the process of something being damaged or deteriorated, particularly in an environmental context.

Wrong options:

• degeneration: 'Degeneration' implies a decline in quality or function, but it is often used in biological contexts rather than environmental, making it less suitable here.
• demotion: 'Demotion' refers to a reduction in rank or status, which does not apply to environmental issues and is irrelevant in this context.
• discredit: 'Discredit' means to harm the reputation of something, which does not relate to environmental challenges and is not applicable in this context.

Correct answer: source

Explanation: The word 'source' is appropriate as it indicates that plants are the origin of energy production, fitting well within the context of discussing their role in food production and energy.

Meaning: 'Source' refers to the origin or starting point of something, in this case, energy production derived from plants.

Wrong options:

• cause: 'Cause' implies a reason for something happening, but it does not specifically denote the origin of energy production as 'source' does.
• root: While 'root' can imply origin, it is more commonly associated with plants themselves rather than the broader concept of energy production.
• outcome: 'Outcome' refers to the result of a process, which does not fit the context of discussing plants as the origin of energy.