Astronomy current events question: past impact on Iapetus?

Astronomy 210L, fall semester 2011
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!)

Robert T. Gonzalez, "Astronomers Say Saturn’s Moon Iapetus May Have Once Had a Mini-moon All Its Own," October 17, 2011
http://io9.com/5848485/astronomers-say-saturns-moon-iapetus-may-have-once-had-a-mini+moon-all-its-own

Researchers at the Southwest Research Institute have hypothesized that a past impact on Saturn's moon, Iapetus, may have created:
(A) Iapetus' equatorial "walnut" ridge.
(B) Saturn's outermost ring.
(C) the nucleus of comet Elenin (C/2010 X1).
(D) minute amounts of dark matter.
(E) Iapetus' thick methane atmosphere.

Correct answer: (A)

Student responses
Sections 70178, 70186, 70200
(A) : 39 students
(B) : 4 students
(C) : 1 student
(D) : 1 student
(E) : 0 students