【C4】A．keepB．maintainC．retainD．protect

【B5】A．keepB．keepingC．keptD．to keep

【C9】A．obtainB．gainC．retainD．maintain

【C13】______A．changeB．maintainC．increaseD．reduce

A．keepB．makeC．takeD．hold

【C9】A．keepB．doC．showD．play

Cooling makes the engine metals their mechanical properties.A.RemainB.to remainC.retainD.t