Astronomy quiz question: water ice on Mercury?

Astronomy 210L, spring semester 2012
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!)

Paul Rincon, "Mercury Poles Give Up Hints of Water Ice," March 22, 2012
http://www.bbc.co.uk/news/science-environment-17470151

NASA's MESSENGER spacecraft revealed that patches of previously-observed bright radar reflections, which may be evidence of water ice on Mercury, coincide with the locations of:
(A) permanently shadowed polar craters.
(B) deeply-buried lava tubes.
(C) ancient dry lakebeds.
(D) underground pockets of dry ice.
(E) midocean rises and subduction zones.

Correct answer: (A)

Student responses
Sections 30678, 30679, 30680
(A) : 37 students
(B) : 3 students
(C) : 6 students
(D) : 3 students
(E) : 1 student