承磨环用青铜制造，环分为2～4段敲入环槽中，且应使承磨环外圆面较活塞外圆面高出_______mm

单项选择题During sea trial, the main engine will be kept run at______load

单项选择题The correct tightening is obtained by the use of______to measure the extension given to the bolts.A.pressure gaugesB.thermometersC.stretch gaugesD.bridge gauges