问题
选择题
_____ is estimated is that 300 million farmers will become urban dwellers in three decades.
A．It
B．As
C．That
D．What
答案
答案：D
考查主语从句。What is estimated是一个主语从句，what引导起这个主语从句，并在句中做主语。AB不能引导主语从句，C在主语从句中不充当任何成分。