What is an oxpecker bird eats insects off the back of a rhinoceros

Actually when the oxpecker bird eats insects off the back of the rhinoceros. They are helping each other. The bird gets a nutritious snack while the rhinoceros gets those pesty and annoying bugs off of him. This is also called a Symbiotic relationship. When both organisms of different species work together to benefit the other. 
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C) the increase in atmospheric pressure pushes the walls of the balloon outward thus making it  is the answer.hope i you.