It was totally by chance they discovered the entrance to the underground palace.A. whenB.

问题补充：

It was totally by chance they discovered the entrance to the underground palace.

A. when

B. which

C. why

D. that

答案：

A. when

B. which

C. why

(答案→)D. that

解析：句意：他们是偶然发现地下宫殿的入口的。这里用的是强调句，强调结构是：It is/was+强调部分+that…，判断是否是强调句的方法是：去掉强调结构，如果这句话仍然成立，就是强调句，这句话如果去掉强调结构是：They discovered the entrance to the underground palace totally by chance.所以是强调句，所以选D。