单项选择题
A.a not knowing
B.an unknowing
C.an unknown
D.a not known
单项选择题 Anchors are prevented from running out when secured by the（）.
单项选择题 An LWT anchor often has difficulty tripping in（）.
单项选择题 An integral part of a blowout preventer that serves as the closing element on an open hole，and whose ends do not fit around the drill pipe but seal against each other and shut off the space below completely is the（）.