下列四组物质中，前者属单质，后者属化合物的是（　　）

A．氧气、空气

B．液氧、石灰水

C．水、高锰酸钾

D．液氮、冰水混合物

This dress is prettier, but it costs ______ that one.

A．twice more than

B．twice as much as

C．as much twice as

D．twice so much as