Academic writing is relatively formal. In general this means that in an essay you should avoid the following:
选项：

A:colloquial words and expressions (such as "stuff", "a lot of", "thing", "sort of");
B:abbreviated forms (such as "can't", "doesn't", "shouldn't”);
C:two word verbs (such as "put off", "bring up”);
D:asking questions (such as “So why is this?”)