A drilling bit which is doughnut shaped to permit recovery of the center portion of the hole drilled is called a（）.

A.A flashing amber light
B.A fixed red light
C.A white flag raised and lowered vertically
D.A flashing white light

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

A.heavy
B.easily damaged
C.affected by temperature
D.costly relative to the strength ratio