Astronomy current events question: tidal heating maintains planet habitability?

Astronomy 210L, fall semester 2014
Cuesta College, San Luis Obispo, CA

Students are assigned to read online articles on current astronomy events, and take a short current events quiz during the first 10 minutes of lab. (This motivates students to show up promptly to lab, as the time cut-off for the quiz is strictly enforced!)

Peter Kelley, "Companion Planets Can Increase Old Worlds’ Chance At Life," July 31, 2014
http://www.washington.edu/news/2014/07/31/companion-planets-can-increase-old-worlds-chance-at-life/

University of Washington and University of Arizona astronomers propose that outer planets use __________ to keep inner planets habitable longer.
(A) reflected sunlight.
(B) solar flares.
(C) greenhouse gases.
(D) magnetic fields.
(E) gravitational forces.

Correct answer: (E)

Student responses
Sections 70178, 70186
(A) : 8 students
(B) : 3 students
(C) : 1 student
(D) : 2 students
(E) : 27 students