These piston rings are so badly worn that they must be （）.
举一反三
- The piston rings are manufactured with a diameter （） the bore of the cylinder.
- On the factory floor, protective hats and boots_______. A: must be worn B: must worn C: must to be worn
- Among the rings of a piston the （） one is subjected to the greatest load and temperature.
- In western culture, wedding rings are worn on right hand.()
- Either a low injection pressure or a leakage of piston rings is able to lead （）.