The Currently Accepted Reasoning for Why the Cosmic Background Radiation

Multiple Choice

The currently accepted reasoning for why the cosmic background radiation is so uniform in temperature is because

A) this temperature uniformity was achieved in the moments before cosmic inflation.
B) as the universe has had time to cool,the hotter parts transferred their heat to the cooler regions.
C) microwaves can only function at a certain very limited range of temperature.
D) space is a vacuum and thus cannot retain heat very well.