Friction is a force
a. that two objects move faster when they are touching.
b. that operates against motion when two objects are touching.
c. that gets stronger when two objects move away from each other.
d. that exists between any two objects in the universe.
need

B. That operates against motion when two objects are touching.

the skeletons of most mammals reach a certain size and then stop growing. however, many animals, including some mammals, keep growing throughout their lives. kangaroos, for example, just keep growing and growing until they die. also, the longer an animal lives the greater the chance it might catch some deadly disease.