Astronomy current events question: fifth gas giant in solar system?

Astronomy 210L, fall semester 2011
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!)

Alasdair Wilkins, "Saturn's Moon Enceladus Spreads Its Influence," September 20, 2011
http://io9.com/5842013/our-solar-system-might-have-once-had-an-extra-gas-planet

A Southwest Research Institute researcher used __________ to infer that our solar system may have formed with a fifth gas giant.
(A) residual photons.
(B) ancient Egyptian star charts.
(C) infrared observations.
(D) computer simulations.
(E) long-range radar pulses.

Correct answer: (D)

Student responses
Sections 70178, 70186, 70200
(A) : 4 students
(B) : 2 students
(C) : 5 students
(D) : 14 students
(E) : 0 students