Oxpecker birds eat parasitic ticks off the backs of zebras is an example of mutualism.
Mutualism is defined as a relationship between organisms in which both the individuals involved are benefitted from each other.
In the case of oxpecker birds and zebras, the oxpecker birds benefit as they get their food from the parasitic ticks living on the zebra and the zebras benefit as the birds help them to get rid of the parasites.
In this way, both these organisms are involved in a mutualistic relationship.
<span>The different adrenal hormones show selectivity for different types of adrenergic receptors. This may be because the chemical structures of the adrenal hormones are such that binding is biochemically easier for the hormones at the receptors to which they show a selective affinity.</span>
The answer is : D) It attracts Us but it is not strong enough for us to feel it. That is because we are too far away. If we were closer to Mars, we would be able to feel the force of attraction.