Carbon dioxide
Hope this helps!
Answer: A dichotomous key is a tool that allows the user to determine the identity of items in the natural world, such as trees, wildflowers, mammals, reptiles, rocks, and fish. Keys consist of a series of choices that lead the user to the correct name of a given item.
Explanation: google
The dependent variable would be the heart rates of her classmates.
The answer is atomic color
I believe youre looking for a double helix if this is an analogy for DNA