Other genetic similarities suggested they all a single, mon ancestor about 300 years ago.
A. had
B. have had
C. have
D. had had
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(答案→)A. had
B. have had
C. have
D. had had
解析：A 考查动词时态。suggest在该句中意为“表明，暗示”，而非“建议”，所以宾语从句中不用虚拟语气。因为宾语从句的时间状语为about 300 years ago，故该句用一般过去时态，选A项。