___ you return those books to the library immediately you will h

___ you return those books to the library immediately you will have to pay a fine.
A、Until
B、Unless
C、If
D、Provided
【正确答案】：B
【题目解析】：unless作连词引导条件状语从句，相当于if not，意为“除非……；如果不……，就会……”，主句多为否定句，如：Jane won't pass chemistry test unless she works hard．除非简努力学习，否则她就不会通过化学考试。