It was totally by chance they discovered the entrance to the underground palace.
A. when
B. which
C. why
D. that
网友回答
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。