Fact 1: Most genes contain the instructions for a single protein.

Fact 2: Genes are the basic unit of heredity.

Given the two facts above, answer the question "What mostly contain the instructions for a single protein?" with the following options: 
- resistance activities
 - Veins and arteries.
 - A Greek letter
 - sensory neurons
 - fur seals
 - parasites
 - units of heredity
 - colors of the spectrum


 

       units of heredity  