Fact 1: Testosterone stimulates the reproductive organs to develop into male organs.

Fact 2: Male mammals have a tube-like organ called the 'penis'.

Given the two facts above, answer the question "What causes the penis to develop?" with the following options: 
- Creatine
 - Energy
 - Estrogen
 - Testicles
 - matter vibrating
 - Testosterone
 - movement
 - It expands


 

      Testosterone   