We Believe the Rate of Bombardment of Projectiles Striking Bodies

We believe the rate of bombardment of projectiles striking bodies in the inner solar system decreased steadily except for a sharp increase between 3.8 and 4.0 billion years ago. What is believed to be the cause of this increase?

A) The Sun flared up and disturbed the entire solar system.
B) The planet that once existed between the orbits of Mars and Jupiter was torn apart leaving the asteroid belt and much new debris to bombard the inner planets.
C) Jupiter and Saturn were moving to their present orbits, and their gravitational fields disturbed the asteroid belt and sent many objects into the inner solar system.
D) The star Proxima Centauri (still the closest star to the Sun) passed close to the solar system and pulled many of the asteroids out of their orbits.