A young man should be ____ his guard against bad company.

A young man should be ____ his guard against bad company.
A、in
B、on
C、from
D、with
【正确答案】：B
【题目解析】：固定搭配 be on one's guard against 某人提防某人，题意为，年轻人应该提防结交坏朋友。