Astronomy current events question: my so-called "equinox"

Astronomy 10L, Spring Semester 2007
Cuesta College, San Luis Obispo, CA

Students are assigned to read online articles on current astronomy events (skytonight.com, from Sky & Telescope magazine), 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!)

[0.2 points.] The 2007 vernal equinox occured on Tuesday, March 20 for San Luis Obispo, CA, signaling the start of spring. Why weren't there exactly 12 hours between sunrise and sunset on that date?
(A) The Earth's orbit around the Sun is not a perfect circle.
(B) Daylight savings time shifts astronomical events one calendar day ahead.
(C) The date of the vernal equinox is misaligned because of accumulated leap year days.
(D) Sunrise and sunset times are defined when the top edge of the Sun is on the horizon, and not when the center of the Sun is on the horizon.
(E) The start of spring in the northern hemisphere marks the start of fall in the southern hemisphere, on the other side of the International Date Line.

Correct answer: (D).

Student responses
Section 4137
(A) : 3 students
(B) : 3 students
(C) : 2 students
(D) : 13 students
(E) : 0 students

Section 4138
(A) : 2 students
(B) : 2 students
(C) : 0 students
(D) : 13 students
(E) : 4 students

Section 4139
(A) : 2 students
(B) : 1 student
(C) : 1 student
(D) : 6 students
(E) : 4 students

When prompted, students know that the "equinox" refers to equal hours of day and night, thus exactly 12 hours between sunrise and sunset. However, on March 20, 2007 for San Luis Obispo, CA, the U. S. Naval Observatory times for sunrise and sunset are:

Sunrise 07:07
Sunset 19:15

This is due to how sunrise and sunset are defined, which is not when the center of the Sun is on the horizon, but when the very top edge of the Sun is on the horizon.