These acid residues must be prevented from entering the crankcase, otherwise the lube oil would be .

He won the scholarship. He ______ hard these terms.A．should have workedB．must have workedC

The soda－acid portable extinguishers must be discharged and refilled _____.A.every six mon

You can exercise your imagination, but these exercises must be done in accordance with the

－－－ Was that the headmaster who walked by？ －－－ _______.A、It must be thatB、It must be him

When we are in a social gathering, we must be aware of the rules and procedures that govern the way things are going to proceed. These rules and procedures are technically called（).

New drugs or devices must be tested before being