The passage implies that the positive-displacement pump differs from the centrifugal pump in that the positive-displacement pump A. is more promising, but it also is more expensive and demands more maintenance B. is especially well research, since it has been used in other settings C. involves the use of a single or twin screw that sucks fluid in at one end of the pump D. is problematic because it cause rapid shifts from liquid to gas content in crude E. involves exposure of many parts of the pump to crude