17. Gel electrophoresis separates fragments of DNA using:
a. a sieving medium through which DNA fragments are moved by gravitational forces
b. repulsion of charged DNA molecules by the electrically charged gel beads
c. a sieving medium through which DNA molecules are moved by electrical forces
d. movement through a sieving medium by positively charged DNA molecules attracted by a negative pole
e. movement of DNA fragments of different sizes at different speeds as a result of ultracentrifugation