Astronomy quiz question: Venus/Mars at sunset?

Astronomy 210 Quiz 2, fall semester 2012
Cuesta College, San Luis Obispo, CA

Consider Earth, and the positions of other planets as shown in the diagram below (not to scale, and orbits have been simplified as circles instead of ellipses).

[Version 1]
If observers in San Luis Obispo, CA observe Venus low over the west horizon at sunset, Venus must be located at:
(A) position (A).
(B) position (B).
(C) position (C).
(D) position (D).
(E) (None of the above choices.)

Correct answer: (B)

An observer at sunset will be located at the top edge of Earth on this diagram, and a horizontal line drawn passing through the top edge of Earth corresponding to the horizon, with planet locations (A) and (B) above the horizon, and thus visible at sunset, while planet locations (C) and (D) would lie below the horizon, and thus not be visible at sunset.  Since the sun would be on the west horizon at sunset, Venus must be placed at location (B) to be just above the sun on the west horizon as well (while a planet at location (A) would be just above the east horizon at sunset.)

Section 70158
Exam code: quiz02k3P7
(A) : 6 students
(B) : 12 students
(C) : 7 students
(D) : 8 students
(E) : 2 students

Success level: 41% (including partial credit for multiple-choice)
Discrimination index (Aubrecht & Aubrecht, 1983): 0.30

[Version 2]
If observers in San Luis Obispo, CA observe Mars low over the east horizon at sunset, Mars must be located at:
(A) position (A).  
(B) position (B).
(C) position (C).
(D) position (D).
(E) (None of the above choices.)

Correct answer: (A)
An observer at sunset will be located at the top edge of Earth on this diagram, and a horizontal line drawn passing through the top edge of Earth corresponding to the horizon, with planet locations (A) and (B) above the horizon, and thus visible at sunset, while planet locations (C) and (D) would lie below the horizon, and thus not be visible at sunset.  Since the sun would be on the west horizon at sunset, Mars must be placed at location (A) to be just above the east horizon (while a planet at location (B) would be just above the sun, on the west horizon at sunset.)

Section 70160

Exam code: quiz02n3Ap
(A) : 11 students
(B) : 8 students
(C) : 10 students
(D) : 1 student
(E) : 1 student

Success level: 42% (including partial credit for multiple-choice)

Discrimination index (Aubrecht & Aubrecht, 1983): 0.06