Don't call me at the office ________ it's absolutely necessary.

Don't call me at the office ________ it's absolutely necessary.
A、except
B、unless
C、since
D、if
【正确答案】：B
【题目解析】：except除了；unless除非；since自从；if如果句意：除非绝对必要，否则不要给我办公室打电话。