Milk will turn sour soon on hot days it is stored in the refrigeratorA. sinceB. untilC.

问题补充：

Milk will turn sour soon on hot days it is stored in the refrigerator．

A. since

B. until

C. because

D. unless

答案：

A. since

B. until

C. because

(答案→)D. unless

解析：句意：牛奶在热天可能会变酸，除非在冰箱里面储存。这里使用连词unless“除非，如果不”，since“自从”，because“因为”，until“直到”，所以选D。