Why Is Thermal Escape of Atmospheric Gas Much Easier from the Moon

Why is thermal escape of atmospheric gas much easier from the Moon than from Earth?

A) The Moon's average surface temperature is lower than Earth's.
B) The Moon lacks a global magnetic field,while Earth has one.
C) Outgassing on the Moon releases gases with lower masses than does outgassing on Earth.
D) The Moon's gravity is so much weaker than Earth's.