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which of the following is a valid statement?
a. formal writing expects the writer to use only compound or complex sentences.
b. academic papers should never use simple sentences.
c. compound sentences should be turned in to complex sentences when possible.
d. good writing includes a variety of sentence structures, including simple, compound and complex sentences.

D. Good writing includes a variety of sentence structures, including simple, compound and complex sentences.

The sentence included here above is true.
Writing includes different structural forms of a sentence however, it also depends on the type of paper being written, for example, academic papers are somehow inclined to simpler sentences than compounded and complex ones. This is because the author does not want to confound and easily captivate the point of the paper to the reader.