Increasing your vocabulary means
A. you'll spend less time on revision.
B. you'll be able to spend more time reading.
C. you'll write more lively sentences.
D. you'll write longer sentences.
I'd say the answer is C, because if you know a lot of words, you can use a whole variety of different words, and synonyms, and antonyms, and what not, and that way you will surely make your sentences more lively and interesting.
