Astronomy quiz question: combining Large Binocular Telescope mirrors into one?

Astronomy 210 Quiz 3, spring semester 2014
Cuesta College, San Luis Obispo, CA

The two optical reflector telescopes of the Large Binocular Telescope located at Mt. Graham, AZ have the same diameters. If these two telescope mirrors were combined into a single mirror with the same total surface area, the light-gathering power would:
(A) decrease.
(B) remain the same.
(C) increase.
(D) (The telescope would no longer be able to function.)

(Image credit: Large Binocular Telescope/NASA, http://www.nasa.gov/topics/universe/features/lbti20101206-i.html.)

Correct answer: (B)

The light-gathering power of a telescope depends on its area, which would be the same if these telescope mirrors were separate, or combined into a single mirror.

Section 30674
Exam code: quiz03n0T7
(A) : 2 students
(B) : 20 students
(C) : 2 students
(D) : 3 students
(No response) : 1 student

Success level: 74% (including partial credit for multiple-choice)
Discrimination index (Aubrecht & Aubrecht, 1983): 0.44

Section 30676
Exam code: quiz03SL3w
(A) : 11 students
(B) : 23 students
(C) : 8 students
(D) : 2 students

Success level: 55% (including partial credit for multiple-choice)
Discrimination index (Aubrecht & Aubrecht, 1983): 0.32