— Shall Mary come to the concert with us？－Notshe has finished her paperA. sinceB. onceC.

问题补充：

— Shall Mary come to the concert with us？

－Notshe has finished her paper．

A. since

B. once

C. when

D. unless

答案：

A. since

B. once

C. when

(答案→)D. unless

解析：

由句意得知，补全句子应该是Mary does not come to the concert with us unless she has finished her paper.