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（）.

单项选择题 An example of a modern anchor which has a stock is a(n)（）.