When the Twin Keck Telescopes Were Built in the 1990's

When the twin Keck telescopes were built in the 1990's, what distinguished them from other very large telescopes of the time?

A) they were reflectors, while other large telescopes were all refractors
B) they were on a mountain, while all other telescopes had been built near big cities
C) they used a mirror assembly that was made of 36 smaller hexagonal mirrors working together
D) they had a smaller overall aperture, which made it easier to take pictures
E) they had a motorized drive system to allow the telescope to move smoothly