Fact 1: Ablack hole is formed by a high-mass star collapses.

Fact 2: Then, gravity collapses the star.

Given the two facts above, answer the question "What is the initial cause of black holes?" with the following options: 
- energy
 - gravity
 - heat
 - photons
 - mechanical
 - sun's heat
 - cracking
 - Smoking


 

  gravity       