高手帮忙：That was written in 1946, since------the education syst

高手帮忙：That was written in 1946, since______the education system has undergone great changes.A then B when C there D which 选B.可我不明白为什么不选A呢,since then 不是加现在完成时吗? 比如 I visited China in 1900. since then, china has undergone great changes. 不是很正常吗?给我详细说说吧,我都失望了,那么多人都觉得不屑的问题我咋就不懂呢.