The look on his face suggested that he just happened the secret and he himself was very

网友回答

A. knowing; 不填
B. to know; 不填
C. knowing; that
(答案→)D. to know; that
解析：考查词组和宾语从句：句意：他脸上的表情表明他只是碰巧知道这个秘密的，他也很吃惊。Happen to do碰巧做…，第二空是宾语从句的连接词that，当有两个宾语从句的时候，第一个宾语从句的that可以省略，但是第二个不能省略。选D。