——This wall of air is an effective answer to the problem and it work. ——But it is a quest

问题补充：

——This wall of air is an effective answer to the problem and it work.

——But it is a question to put such a wall of air around the statue.

A. may

B. must

C. should

D. could

答案：

A. may

B. must

C. should

D. could

解析：本题主要考查情态动词的用法。选项A表示事实上的可能或允许；选项B表示要求或肯定的推测；选项C表示建议或根据一般情况的推测；选项D表示较小的可能性。根据句子中的关键词an effective answer可知，后面表示在一般情况下的推测。