问题
选择题
Tony ________ full marks, but he was so careless as to make a spelling mistake.
A．could have gained
B．can have gained
C．must have gained
D．must gain
答案
答案：A
题目分析：句意：Tony本来可以得满分的，但是他太粗心了，犯了一个拼写错误。could have gained表示过去本来可以做的事而没做；can have gained表示对过去情况的怀疑或不确定；must have gained表示对过去发生的事情或状态进行推测，选A。