Why Did the Solar Nebula Heat Up as It Collapsed

Why did the solar nebula heat up as it collapsed?

A) Radiation from other nearby stars that had formed earlier heated the nebula.
B) The shock wave from a nearby supernova heated the gas.
C) Collisions among planetesimals generated friction and heat.
D) Nuclear fusion occurring in the core of the protosun produced energy that heated the nebula.
E) As the cloud shrank, its gravitational potential energy was converted to thermal energy.