The indoor swimming pool seems to be a great deal more luxurious than ______.

A.is necessary 

B.being necessay 

C.to be necessary 

D.it is necessary

答案说选A.

   【答】than 和 as 在从句中可以替代主语（很像关系代词）。

He worries more than is necessary. 

他担心过度。（不能说：...more than it/what is necessary.）

There were a lot of people at the exhibition — more than came last year. 展览会上有许多人 — 比去年来的人要多。（不能说：...more than they came last year.）

（摘自《牛津高阶英汉双解词典》第9版256.1节）