Astronomy current events question: asteroid impact spherules

Astronomy 210L, spring semester 2012
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!)

Astronomy.com editors, "ASA Scientists Find History of Asteroid Impacts in Earth Rocks," April 26, 2012
http://astronomy.com/en/News-Observing/News/2012/04/NASA%20scientists%20find%20history%20of%20asteroid%20impacts%20in%20Earth%20rocks.aspx

Thin layers in ancient rocks from Australia that contain __________ provides evidence that some 70 large asteroids have impacted Earth 1.8 to 3.8 billion years ago.
(A) moon fragments.
(B) radioactive elements.
(C) cometary ice.
(D) fossils of extinct life forms.
(E) hardened molten droplets.

Correct answer: (E)

Student responses
Sections 30678, 30679, 30680
(A) : 2 students
(B) : 3 students
(C) : 3 students
(D) : 1 student
(E) : 8 students