The gap between rural and urban education in China is widening, and education is the key to _____ rural problems.

A．solving

B．solve

C．have solved

D．having solved

停运真空泵前应（）

A.关闭抽气阀

B.关闭排气阀

C.关闭循环水阀

D.打开气水分离器放水阀