Does Liu Hui serve in the army? No, but he has served in the army for three years.
做题要找到“题眼”，依据。此句“题眼”，是for three years。谓语动词一定是完成时态。
B. has served
for three years.表示持续状态，用现在完成时
B.
因为服役的事情已经完成 ，所以要选现在完成时。
for加一段时间要用现在完成时,所以选B
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