Which of the following describes a competitive relationship between two organisms?

A.
A deer tick lives on a white-tailed deer and drinks the blood of the deer.
B.
An owl and a snake in the same ecosystem both eat mice as a major food source.
C.
An oxpecker sits on the back of a zebra to pick small parasites off the zebra's back.
D.
A bluebird swoops down from the air to catch a grasshopper and eats it.