Which Reason Best Explains Why Both Venus' and Mars' Atmospheres

Which reason best explains why both Venus' and Mars' atmospheres are primarily carbon dioxide, but Earth's is much less than 1% carbon dioxide?

A) Earth's rocks had much less carbon and oxygen in them.
B) Venus and Mars, being smaller than Earth, can't retain the same mixture of gases as Earth.
C) Each planet has a significantly different surface temperature.
D) Plant life has existed on Earth for an extended amount of time.