—You look so worried, what’s wrong?

—More and more people like me become to worry about the __________ pollution  after the haze (雾霾) appeared in Beijing and some other cities in China.

A．water

B．air

C．noise

D．Soil

常用的关系运算是关系代数和

A) 集合代数
B) 逻辑演算
C) 关系演算
D) 字段