Several pages were found -----when the book was returned to

问题补充：
我觉得该选A,可答案选的是C.我还是想不明白……
C
were found gone 被发现缺页.强调的是不翼面飞.
A missing 强调不见.丢失.显然是gone 好些,很是讽刺.
lost 是不行的,用被动,不当,因为强调不是被丢,而是强调不见了.