某农场的小麦生长不茂盛，叶色枯黄，经咨询需要施加氮肥．下列物质属于氮肥的是（　　）

A．K₂SO₄

B．Ca（H₂PO₄）₂

C．KCl

D．CO（NH₂）₂

Look! There are so many students         in the playground.

A. playing B. plays  C.play