Astronomy quiz question: IAU classification of Ceres with oceans in the past

Astronomy 210 Quiz 4, spring semester 2018
Cuesta College, San Luis Obispo, CA

Refer below to the minimal qualifications established by the International Astronomical Union for a planet:
I. Orbits the sun.
II. Shape "rounded-out" by gravity.
III. Cleared/dominates orbit around sun.
Observations from NASA's Dawn spacecraft indicate that the dwarf planet Ceres may have had an ocean of water in its past[*]:
Today, Ceres is a salt-covered dwarf planet whose main claim to fame is that it's the largest body in the main asteroid belt. But back when it was younger and hotter, scientists have found Ceres was an ocean world—much like the watery moons of Jupiter and Saturn.  It's also possible the world is still alive and venting water vapor into space, that icy volcanism is continually resurfacing parts of it.
According to the IAU qualifications, when Ceres had oceans in its past, it would have been classified as:
(A) a moon.
(B) solar system debris.
(C) a dwarf planet.
(D) a planet.
(E) (None of the above choices.)

[*] Nadia Drake, "Ceres, the Dwarf Planet Formerly Known as an Ocean World," phenomena.nationalgeographic.com/2016/03/23/ceres-the-dwarf-planet-formerly-known-as-an-ocean-world/.

Correct answer (highlight to unhide): (C)

As a dwarf planet today, Ceres directly orbits the sun (meeting qualification I), has a rounded shape (meeting qualification II), but does not satisfy qualification III (as it shares its orbit around the sun with the asteroids). Having an ocean in its past would not have affected whether it met or did not meet these qualifications, such that it would have still been classified as a dwarf planet.

Section 30674
Exam code: quiz04NCh4
(A) : 7 students
(B) : 0 students
(C) : 12 students
(D) : 5 students
(E) : 0 students

Success level: 55% (including partial credit for multiple-choice)
Discrimination index (Aubrecht & Aubrecht, 1983): 0.83

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