—The old man is ill. He ________ to the hospital at once.

--Don’t worry. A doctor _________to come already.

A．must be sent , has asked

B．must send, have asked

C．must be sent, has been asked

D．must send, have been asked

二氧化碳可以用来灭火，也可使用石灰桨抹的墙变得坚硬．其中的化学原理分别是什么？