单项选择题
A drilling bit which is doughnut shaped to permit recovery of the center portion of the hole drilled is called a（）.
A.diverter bit
B.hole reamer
C.core bit
D.conventional bit
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A.A flashing amber light
B.A fixed red light
C.A white flag raised and lowered vertically
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A.at all times
B.in the bottom return mode
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B.easily damaged
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