A difference between a morula and a blastula is that the:

A) morula is a solid ball of tightly packed cells, whereas the blastula has a cavity in the center.
B) blastula has fewer cells than the morula.
C) morula has a hollow center, but the blastula doesn't.
D) morula is a well-developed embryo, and the blastula is an earlier stage of development.