Astronomy current events question: sudden appearance of "Pinnacle Rock"

Astronomy 210L, spring 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!)

Ian O'Neill, "Mystery Rock 'Appears' in Front of Mars Rover," January 17, 2014
http://news.discovery.com/space/mystery-rock-appears-in-front-of-mars-rover-140117.htm

The Mars Exploration Rover Opportunity observed the sudden appearance of the small white rock "Pinnacle Island" that may have been moved by:
(A) one of its wheels.
(B) a recent marsquake.
(C) evaporating dry ice snow.
(D) a lightning strike.
(E) receding floodwaters.

Correct answer: (A)

Student responses
Sections 30678, 30679, 30680
(A) : 36 students
(B) : 7 students
(C) : 3 students
(D) : 5 students
(E) : 5 students