DNA is common in ALL organisms. What about the DNA makes one species different than another?

All DNA molecules in both plants and animals are made from the same four chemical building blocks called nucleotides.

Explanation:

But because the DNA coping process is imperfect, mistakes accumulate over time and making cytochrome C slightly different in different creatures.

i think it would be b. tissue because an organ system is a group of organs that work together to perform one or more functions. each does a particular job in the body, and is made up of certain tissues.