To improve our sentence fluency, we should be careful to always. a. give every last detail we know about a subject.
b. always write sentences that have similar word count.
c. use the most complex vocabulary possible.
d. vary the complexity of our sentences.

c

Explanation:

d. vary the complexity of our sentences

- "the basic difference between keywords and subjects is that subjects (or "subject headings") have been predetermined, while keywords have not. a keyword is an easy place to start because you can use any word that seems to fit your search; this is called "natural language" searching."

i feel like it is actually b.