单项选择题
The crankcase cover should never be allowed to open for investigation when ()
	A.the engine stops
	B.the engine is running
	C.you repair the engine
	D.overhauling the engine
                    
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                                    单项选择题
                                    
The crankcase can not be opened ()the parts have been cooled down.
A.if
B.as
C.unless
D.when - 
                                    单项选择题
                                    
The crank throws are displaced from () to obtained the same number of degrees between the different crank throws.
A.one another
B.the other
C.each other
D.each others - 
                                    单项选择题
                                    
The crank pin journal is provided with large bores for ().
A.lubrication
B.cooling
C.balancing purpose
D.ventilation