How does a phase-contract microscope setup make transparent objects more visible?

A)It uses brighter bulbs than conventional light microscopes.
B)It generates a negative image so that the background is dark and the specimen is light.
C)It generates stereo images that can be viewed only with polarized filters.
D)It causes direct and refracted light rays to interfere with one another.
E)It operates at a lower temperature to reduce random specimen movement.