Test seven18.only when it rains ------

答案是A.原因是句子以only开头,主句要进行半倒装.所谓的半倒装就是不分谓语要提到主语的前面.另外是一些表示否定意义的词位于句首时主句要进行半倒装.
这里的谓语动词如果不倒装就是overflows,可是要部分提前的话,得给它加和其时态、语态一致的助动词,所以构成了does ...overflow; 如果是过去时,则变为did...overflow.
另外的例子：
Never did I think he would say such things to me.
Seldom does my Mom criticize me for low scores.
Not only did he eat the egg,but also he ate the rice.
Only when you realize the need for money will you appreciate what your parents have done for you.