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SAT 真题语法问题While studying mutations in corn,biologist Barbara

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SAT 真题语法问题
While studying mutations in corn,biologist Barbara McClintock discovered that certain genes that were able to move their positions on chromosomes and control the activity of other genes.
A that were able
B that had been able
C were able
D able
E to be able
为什麼不是C呢?
要么lz看错了要么答案给错了,C是对的,选B的话从句没有谓语