（） is a precaution which contributes to avoiding trouble under normal operating conditions.

（） are mounted on and bolted to the bed-plate cross girders.

A.The crankshaft
B.The crankcase
C.The guide-ways
D.The frames

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（） are formed by abrasive particles entering the bearing and scoring the shaft.

A.Grooves
B.Recesses
C.Ridges
D.Nips

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