单项选择题 A large vessel is equipped with a controllable pitch propeller. Which statement is TRUE?（）

单项选择题 A large valve，usually installed above the ram preventers，that forms a seal in the annular space between the pipe and wellbore or，if no pipe is present，on the wellbore itself is called the（）.

单项选择题 A large metallic device，mounted directly in the piping (usually located at the dock near the point where the vapor hose is attached)，designed to prevent the passage of a rapidly moving flame through the piping is called a（）.