You are presented with the question "What causes the penis to develop?" and the following answer choices: 
- Creatine
 - Energy
 - Estrogen
 - Testicles
 - matter vibrating
 - Testosterone
 - movement
 - It expands

Now knowing that testosterone stimulates the reproductive organs to develop into male organs and male mammals have a tube-like organ called the 'penis', choose the best answer.


 

      Testosterone   