What Is the Basic Idea Behind the Planetesimal Hypothesis

What is the basic idea behind the planetesimal hypothesis?

A) That planets form as a direct result of the nuclear fusion of nebular gases and planetesimals.
B) That planets form from the remains of super-giant planetesimals that undergo nuclear fission and blow apart,thereby creating smaller objects-the planets.
C) That early in the history of the solar system,a star passed near to the Sun and pulled off gases that eventually condensed to form planets.
D) That small grains of cosmic dust and other solids gradually accrete to form planetesimals that may grow to become protoplanets and eventually planets.