Answer to Question 1
A glance at the Solar System suggests that you should expect to find a planet between Mars and Jupiter at the present location of the asteroid belt. Mathematical models indicate that the reason asteroids are there rather than a planet is that Jupiter grew into such a massive body that it was able to gravitationally disturb the motion of nearby planetesimals. The bodies that could have formed a planet between Mars and Jupiter instead collided at high speeds and shattered rather than combining, were thrown into the Sun, or were ejected from the Solar System. The asteroids seen today are the last remains of those rocky planetesimals.
Answer to Question 2
a