What Is So Unique About the Mirrors of Telescopes Such

Multiple Choice

What is so unique about the mirrors of telescopes such as the Keck Observatory, Gran Telescopio Canaries and the soon-to-be-built Giant Magellan Telescope?

A) The mirrors are made of a newly designed metallic material.
B) The mirrors can fold up went not in use.
C) The mirrors are inexpensive.
D) The mirrors are segmented.
E) The mirrors are thin.