By the time you get to Greenwich you__ the most historic parts of London.

正确答案：选D.will have seen
即：By the time you get to Greenwich you will have seen the most historic parts of London.
翻译：你一到格林尼治你就已经看到伦敦最具有历史意义的地区了.
解析：看一下下面几个例句
1 By the time she COMES tomorrow,we will have done our work.(将来)
2 By the time she CAME last week,they had done the work.（过去）
3 By this time(=now) ,all the students HAVE gone home (到眼下)
所以如果by the time后面接的表示将来的时间,那么主句用将来完成时,所以选D
有不明白的请继续追问,