On East Africa s Serengeti Plains, herbivores migrate across this area following

Grasses! The north and west crossing the Grumeti River, There where more grass.

The correct answer is A. The solar nebula flattened into a rotating disk. As gas became dense and hot, then it spins faster and pulled towards the center whereby the sun is formed.

Solar nebula they collapse where the protostellar disk rotates. In the center of of nebula, there is a fusion begins and then sun is being formed.

The other molecular clouds they flattened to protoplanetary disk whereby the asteroids, planets, moons, and other small systems of solar are being formed.