A heavy，thick-walled tube，usually steel，used between the drill pipe and the bit in the drill stem to weight the bit in order to improve its performance is called a（）.

A.main brace
B.tow span
C.jockey bar
D.None of the above

A.one sidelight
B.one sidelight and a masthead light
C.one sidelight and two masthead lights
D.both sidelights of a vessel and her masthead light(s)

A.vernier
B.psychrometer
C.hygrometer
D.stadimeter