Astronomy current events question: billions of inhabitable planets?

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!)

Clara Moskowitz, "Billions of Alien Habitable Planets, Right Here in Our Galaxy," March 28, 2012
http://www.msnbc.msn.com/id/46880631/ns/technology_and_science-space/#.T4yr5-nLw0V

An estimated tens of billions of habitable planets may exist in the Milky Way, based on observations by the European Southern Observatory in Chile of:
(A) dark matter clouds.
(B) extraterrestrial radio transmissions.
(C) reflections from exoplanets with oceans.
(D) slowly rotating medium-mass main-sequence stars.
(E) super-Earths orbiting red dwarfs.

Correct answer: (E)

Student responses
Sections 30678, 30679, 30680
(A) : 2 students
(B) : 2 students
(C) : 1 student
(D) : 4 students
(E) : 16 students