Which of the following best describes why the Moon goes through phases?

a. We see only part of the lit-up half of the Moon depending on its position relative to Earth and the Sun.
b. Earth’s clouds cover potions of the Moon at various times resulting in the changing phases that we see.
c. Earth's shadow falls on different parts of the Moon at different times.
d. The sunlight reflected from Earth lights up the Moon but is less effective when the Moon is lower in the sky than when it is higher in the sky.
e. The Moon is somewhat flattened and disk-like, and appears more or less round depending on the precise angle from which we see it.