C. Whales. Because normally it is the first word and even though mammals would be considered a subject, it isn't the main subject.
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The answer is B. [Im]perceptible means [not] perceivable, and to perceive is to understand.
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Despite the difficulties and circumstances in which the people live, they have not abandoned science because they are certain that they are the fruit of the future, with which they will change the present to a better future.
Citing passages from a text is one way to provide elaboration in an essay.
The reason why this would be your answer is because when you cite your source, it allows to person to go to the source to see more information about the quote you got, and not leave them empty handed. If you didn't cite you source, it would be considered plagiarism, and wouldn't allow the reader to get to know more about the quote and what happens.
