What Do We Mean by the Period of Heavy Bombardment

What do we mean by the period of heavy bombardment in the context of the history of our solar system?

A) the time before planetesimals finished accreting into planets, during which many growing planetesimals must have shattered in collisions
B) the time during which heavy elements condensed into rock and metal in the solar nebula
C) the period about 65 million years ago when an impact is thought to have led to the extinction of the dinosaurs
D) the first few hundred million years after the planets formed, which is when most impact craters were formed