Question

What happens in a cold front? A.A cold air mass catches up to a moving warm air mass, sliding over it. B.A warm air mass becomes a cold air mass as it moves. C.A warm air mass collides with a stationary cold air mass, and slides under it. D.A cold air mass comes in under a warm air mass.

Answer 1

Answer:

A cold air mass comes in under a warm air  mass

Explanation:

Answer 2

In meteorology (the study of weather), a "front" is the leading edge
of an air mass, that's moving in and replacing a mass of a different
kind of air.  Like warm moving in and replacing cold, or cold moving
in and replacing warm.

Remember that cold air is heavier than warm air.  So whenever a
warm or cold front moves in, the cold air always stays low and the 
warm air always lifts up over it.

With that little bit of an explanation, you can see that choice-D is
the only choice that makes sense.