Mark Needs to Learn the Planets of the Solar System

Multiple Choice

Mark needs to learn the planets of the solar system listed in the order of their proximity to the sun: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. If we consider the serial learning curve, which planets will probably be the most difficult for Mark to learn?

A) The planets closest to the sun (e.g., Mercury, Venus)
B) The middle planets (e.g., Mars, Jupiter)
C) The planets farthest from the sun (e.g., Uranus, Neptune)
D) The planets at the two ends of the list (e.g., Mercury, Neptune)