the twins have learned a lot ______ they came to china .连词填空

问题描述:

the twins have learned a lot ______ they came to china .连词填空

before 在来中国之前,这对双胞胎已经了解不少东西了

since
因为主句是现在完成时,用since 作为连词最恰当
如用before 作为连词,主语要用过去完成时