下列物质按照单质、混合物、氧化物的顺序排列的是（　　）

A．水银、硬水、水

B．石墨、熟石灰、生石灰

C．氨气、空气、消石灰

D．冰、醋酸、干冰

. ______ understanding the challenges faced by the world’s poorest people can we expect a new future

free of extreme poverty and hunger.

A．On

B．Through

C．Only

D．Only by