In essence, the nebular theory holds that
- the planets each formed from the collapse of its own separate nebula
- the planets formed as a result of a near-collision between our sun and another star
- our solar system formed from the collapse of an interstellar cloud of gas and dust
- nebulae are clouds of gas and dust in space

C) According to the nebular theory, a solar system begins when an interstellar cloud, containing approximately 75 percent hydrogen, 25 percent helium and traces of other elements, begins to form areas of higher concentration, or clumps. As the clumps grow, gravitational forces increase and get converted to the kinetic energy of the increasingly fast-moving particles, which collide with one another and generate heat.
Seattle PI.com

Answerbecause of improved transportation, communication, and other advantages;

Why you put last one a? i think is b.