When_______help, one often says “Thank you.” or “It’s kind of you”.

A．offering

B．to offer

C．to be offered

D．offered

高压蒸汽灭菌开始之前为什么要将灭菌锅的冷空气排净尽？灭菌完毕后什么情况下才可以打开锅盖取灭菌物品？