The Reason That the Primary Mirror of an Astronomical Telescope

The reason that the primary mirror of an astronomical telescope is often shaped and polished to a parabolic shape is

A) to avoid the chromatic aberration that would be produced by an equivalent spherical mirror.
B) that it is lighter and easier to mount in a telescope because it requires less glass than an equivalent spherical mirror.
C) that it is easier to produce, even though the resulting mirror will produce more spherical aberration than an equivalent spherical mirror.
D) to avoid spherical aberration by bringing parallel rays to a single focus.