问题详情：
Tom full marks, but he was too careless a spelling mistake.
A．could gain, to make B．could have gained, to make
C．could have gained, and made D．could have gained, as to make
【回答】
C
【解析】
考查含情态动词的虚拟语气和时态。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C。
知识点：一般过去时及其被动式
题型：选择题
