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

Fact 2: Testicles are male reproductive organs.

Given the two facts above, answer the question "What stimulates the reproductive organs to develop testicles?" with the following options: 
- sperm
 - mechanical
 - barnacles
 - the penis
 - the moon
 - testosterone
 - Chemical energy
 - competition


 

      testosterone   