The specific subject of a piece of writing is known as a:

A. topic
B. subtopic
C. supporting detail
D. expository text

I think it's C. I don't really know what B and D mean.

C is not correct.

Please check your text for the meanings of these words.

Is subtopic? Because the meaning of subtopic identifies a part of a writing piece.

All of those terms identify part of a written piece.

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The specific subject of a piece of writing is known as the topic. Therefore, the correct answer is A. topic.

To arrive at this answer, you can break down the options and understand their meanings:
A. Topic refers to the main subject or idea of a piece of writing. It is what the text is primarily about.
B. Subtopic is a subcategory or a smaller subject within the main topic. It is more specific and focused.
C. Supporting detail includes specific information or evidence that is used to explain, justify, or develop the topic. It supports and expands upon the main idea.
D. Expository text refers to a type of writing that aims to inform or explain a specific topic. It presents information in a clear and objective manner.

In this case, B. subtopic refers to a subsection or a more specific aspect of the main topic. It is not the correct answer because the question asks for the overall subject of the writing, not a smaller component.

D. expository text is also not the correct answer because it describes a specific genre or type of writing rather than the subject or main idea of a piece of writing.

By process of elimination, you can determine that the correct answer is A. topic.