[D16] ______ you have seen both fighters,______ will win?

Since you have seen both fighters,who do you think will win?既然你看到两方竞争者了,你认为谁会赢?
其实这个句子主句为Who do you think will win ,只是它把时间状语Since you have seen both fighters提前了,{do you think 可以看成是主句的插入语}
这个句子也可以这么说,Who do you think will win since you have seen both fighters?
【since ,既然,由于】
而主句是特殊疑问句Who will win中间插入一个插入语do you think
祝学业进步~

既然你已经看过两名赛手，你觉得哪一个会赢？
因为是一个问句，所以要用who do you think.
前面since是由于，既然的意思，是一个从句，后面应该是一个完整句子，所以要选D

D. since连词意思是“既然”， do you think作插入语。