In hot summer milk quickly turns sour it is refrigerated.A. whileB. ifC. untilD. unless

问题补充：

In hot summer, milk quickly turns sour it is refrigerated.

A. while

B. if

C. until

D. unless

答案：

A. while

B. if

C. until

(答案→)D. unless

解析：考查连词，A. while当…时候B. if如果C. until直到…D. unless除非，引导条件状语从句，句子意思“在炎热的夏季，牛奶很快变酸除非被冷藏”