Astronomy current events question: "J waves" through Earth's solid inner core

Astronomy 210L, fall 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!)

Jessica Fagan, "ANU Researchers Confirm Earth's Inner Core is Solid" (October 19, 2018)
anu.edu.au/news/all-news/anu-researchers-confirm-earth’s-inner-core-is-solid

Analysis of faint "J waves" or shear waves received by paired detectors after major earthquakes provide evidence that Earth's inner core is:
(A) completely solid.
(B) rapidly shrinking.
(C) dynamically unstable.
(D) electrically conductive.
(E) about to flip its magnetic poles.

Correct answer: (A)

Student responses
Sections 70178, 70186
(A) : 19 students
(B) : 1 student
(C) : 3 students
(D) : 3 students
(E) : 1 student