Astronomy current events question: oscillation-ejected stars in Milky Way's halo?

Astronomy 210L, spring semester 2018
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!)

Mari-Ela Chock, "Stars Around the Milky Way: Cosmic Space Invaders or Victims of Galactic Eviction?" (February 26, 2018)
keckobservatory.org/recent/entry/halo_stars

Based on analysis of their chemical composition, groups of stars in our Milky Way galaxy's disk may have been knocked out into the halo from oscillations produced by:
(A) matter and antimatter.
(B) type II supernovae.
(C) a passing dwarf galaxy.
(D) the central supermassive black hole.
(E) dark matter.

Correct answer: (C)

Student responses
Sections 30679, 30680
(A) : 3 students
(B) : 3 students
(C) : 24 students
(D) : 5 students
(E) : 2 students