These boxes for keeping the food clean and cool.A. useB. are usedC. have usedD. were usin

问题补充：

These boxes for keeping the food clean and cool.

A. use

B. are used

C. have used

D. were using

答案：

A. use

(答案→)B. are used

C. have used

D. were using

解析：句意：这些箱子是用来保持食物清洁和保鲜的。结合语境可知本句描述的是客观性动作，故用一般现在时态。主语是动作对象，故用被动语态，选B。