--- It’s the fourth time that heavy smog(雾霾) has hit the city this month.

--- It’s high time we _______ measures to fight air pollution.

A．will take

B．took

C．to take

D．taking

延缓混悬微粒沉降速度最简易可行的措施是

A．降低分散介质的黏度

B．减小混悬微粒的半径

C．使微粒与分散介质之间的密度差接近零

D．加入反絮凝剂

E．加入表面活性剂