环形防喷器在关井状态下允许上下低速活动钻具。
参考答案：对
近似值是12.50的最大三位数______，最小三位数是______．
How many adult life jackets are required on board a ship?（）
A.Enough for 100 percent of the persons allowed on board
B.One for each work station and industrial work site
C.Enough for 150 percent of the persons allowed on board
D.Both A and B above
