B!
Parasitism is when one organism benefits and harms the organism it is living off of, while the organism it is living off of will only be harmed and does not benefit.
There are four principles at work in evolution—variation, inheritance, selection and time. The theory of evolution by natural selection, first formulated in Darwin's book "On the Origin of Species" in 1859, is the process by which organisms change over time as a result of changes in heritable physical or behavioral traits.
That's hard. The action might be when you go to lift your feet off the ground and the reaction force is letting your feet touch the ground again while taking a stride. Hope this helps.
