According to the survey, two thirds of the area__________covered by trees now and 90% of the population__________contributions to the Greening Project in the city.

A．is ; has made

B．are; have made

C．is; have made

D．are; has made

The exhaust pipe must be gas tight throughout its entire length otherwise（）.

A.bilge water may enter the exhaust pipe

B.entry of air may cause vapor lock

C.carbon monoxide may enter the interior of the vessel

D.the joint gaskets may be blown