Astronomy current events question: estimate of Earth-sized planets in habitable zones

Astronomy 210L, fall 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!)

Erik Petigura, Andrew Howard, Geoff Marcy, and Steve Jefferson, "One in Five Stars Has Earth-sized Planet in Habitable Zone," November 4, 2013
http://www.keckobservatory.org/recent/entry/one_in_five_stars_has_earth_sized_planet_in_habitable_zone

As much as 20% of sun-like stars in the Milky Way may __________, based on statistical analysis of data from the Kepler Space Telescope.
(A) undergo type II supernova explosions.
(B) have Earth-sized planets.
(C) be extremely metal-rich.
(D) harbor carbon-based life.
(E) outlive the Milky Way itself.

Correct answer: (B)

Student responses
Sections 70178, 70186, 70200
(A) : 2 students
(B) : 53 students
(C) : 0 students
(D) : 1 student
(E) : 1 student