问题补充：
Tom full marks, but he was too careless and made a spelling mistake.
A. could gain
B. could have gained
C. must have gained
D. must gain
答案：
A. could gain
(答案→)B. could have gained
C. must have gained
D. must gain
解析：句意：汤姆本可以得到满分的，但是他太粗心了，犯了一个拼写错误。从but he was too careless and made a spelling mistake.可知是说明的过去的事情，排除AD，could have done“本可以做而没有做”是虚拟语气，符合句意，must have done过去一定做了，不符合句意，所以选B。
