The teacher ________ have thought Johnson was worth it or she wouldn't have wasted time on him, I suppose.A.shouldB.canC.wouldD.must
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D解析思路分析：从后面的“or she wouldn't have was ted time on him”可知，前面的分句应该表示一种肯定的判断，故选must。句意是“老师一定是认为Johnson值得……”。must have done表示对发生于过去的情况的判断；should应该，表示按事理和常规作判断；can表示一种可能性；would表过去的意愿或趋势。故选D项。
【点评】该题考查情态动词的用法，偏难。情态动词表示一种说话的语气，考生应仔细品味上下文的语意及句子暗示的语气作出选择。