补写下列谚语。     

1．读万卷书，__________________ 。    

2．海内存知己，____________________ 。    

3．千里之行，___________________ 。    

4．台上三分钟，___________________。    

5．欲知山中事，___________________。    

6．瓜熟蒂落，__________________ 。

______ was most shocking about the tornadoes in America was that they threatened six states and killed more than 100 persons.

A．That

B．Which

C．Why

D．What