D, it is the author's belief.
Social duty simply means the responsibility or role that one must play in society.
It should be noted that social duty is also called social responsibility. It means that everyone have a duty to act in the best interest of others.
It's an ethical framework that states that individuals have an obligation to cooperate with others to ensure that the environment is healthy.
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I believe the answer is A.
b. is a prepositional phrase
d. is a complete noun phrase
and I think c. is a verb phrase
