Astronomy current events question: dwarf galaxy NGC 4625's single spiral arm

Astronomy 210L, fall semester 2017
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!)

Karl Hille, "Hubble’s Cosmic Search for a Missing Arm" (November 17, 2017)
nasa.gov/image-feature/goddard/2017/hubbles-cosmic-search-for-a-missing-arm

The single spiral arm structure and young outer region star formation in dwarf galaxy NGC 4625 were caused by gravitational interactions from:
(A) its supermassive black hole.
(B) type II supernova explosions.
(C) intense ultraviolet light.
(D) another dwarf galaxy.
(E) dark matter.

Correct answer: (D)

Student responses
Sections 70178, 70186
(A) : 2 students
(B) : 7 students
(C) : 8 students
(D) : 28 students
(E) : 0 students