Fact 1: Testosterone stimulates the reproductive organs to develop into male organs. Fact 2: Testicles are male reproductive organs. Given the two facts above, what stimulates the reproductive organs to develop testicles?

OPTIONS:
1). sperm;
2). mechanical;
3). barnacles;
4). the penis;
5). the moon;
6). testosterone;
7). Chemical energy;
8). competition;
Answer:
6).