Astronomy current events question: Mercury's magma ocean?

Astronomy 210L, spring semester 2013
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!)

Sarah McDonnell, "Mercury May Have Harbored an Ancient Magma Ocean," February 21, 2013
http://web.mit.edu/press/2013/mercury-may-have-harbored-ancient-magma-ocean.html

MIT scientists have proposed a timeline for Mercury's geological past by analyzing __________ based on data collected from Mercury's surface.
(A) radioactivity levels.
(B) man-made rocks.
(C) impact crater densities.
(D) the lack of polar ice caps.
(E) greenhouse gases.

Correct answer: (B)

Student responses
Sections 30678, 30679, 30680
(A) : 13 students
(B) : 17 students
(C) : 18 students
(D) : 1 student
(E) : 1 student