Fact 1: Nephrons are the structural and functional units of the kidneys.

Fact 2: Each nephron contains a system of blood vessels and tubules.

Given the two facts above, answer the question "what contains a system of blood vessels and tubules" with the following options: 
- clouds
 - kidneys
 - organ
 - density
 - lungs
 - complex
 - blood cells
 - alveoli


 

  kidneys       