农业上可用来中和土壤酸性，改良土壤结构的物质是（　　）

A．石碱

B．烧碱

C．熟石灰

D．氨水

If passengers are on board when an abandon ship drill is carried out，they should（）.

A.take part

B.watch

C.go to their quarters

D.stay out of the way and do what they want