A device used to enlarge the size of an existing bore hole, having teeth arranged on its outside circumference to cut the formation as it rotates is a (n)（）.

A.velocity of the current
B.bottom characteristics
C.depth of the water
D.speed over the ground

A.speed being made good
B.speed through the water
C.the set of the current
D.the depth of the water

A.at all times
B.in the bottom return mode
C.in the volume reverberation mode
D.only when there is no current