When it comes to energy, saying that something is cleaner than coal means that it does not harm the environment as much as burning coal does and it is usually sustainable. The clean is the same one in the term "clean energy"
It compares the number of mutations that exist in the DNA of two different species.
A numerical expression includes numbers and operations, so if youre wanting a numerical expression for the number 20, all you have to do is add, subtract, multiply, or divide numbers to equal that.
For example, 5+15=20, 4*5=20, 80/4=20, 29-9=20
Number 6. Response to stimuli. At least I think. Or 6, homeostasis.