Astronomy quiz question: hottest/coolest star?

Astronomy 210 Quiz 4, spring semester 2013
Cuesta College, San Luis Obispo, CA

[Version 1]
Which star is the coolest?
(A) M5 giant.
(B) F0 white dwarf.
(C) K5 main-sequence star.
(D) O5 main-sequence star.

Correct answer: (A)

Using the OBAFGKM spectral type mnemonic, or reading off of an H-R diagram (provided on this quiz, as below), the O5 main-sequence star is the hottest, followed by the F0 white dwarf, K5 main-sequence star, and M5 giant.

Section 30674
Exam code: quiz04Ni7E
(A) : 19 students
(B) : 2 students
(C) : 5 students
(D) : 2 students

Success level: 69% (including partial credit for multiple-choice)
Discrimination index (Aubrecht & Aubrecht, 1983): 0.50

[Version 2]
Which star is the hottest?
(A) M5 giant.
(B) F0 white dwarf.
(C) K5 main-sequence star.
(D) O5 main-sequence star.

Correct answer: (D)

Section 30676
Exam code: quiz04sGf6
(A) : 3 students
(B) : 3 students
(C) : 1 student
(D) : 35 students

Success level: 84% (including partial credit for multiple-choice)
Discrimination index (Aubrecht & Aubrecht, 1983): 0.18