Well considering birds follow humane rules, their selection and environment around them being thriving could effect their songs in a positive way. While a bird with a low selection of environmental stuff around them would effect their songs in a bad way giving off a more down manner.
The answer is To your question is D
Introduction to the outer condition and conceivable cancer-causing agents and, or the fast rate of mitosis that would enable malignancy to spread rapidly.
The skin has body securing capacities, it shields the interior organs from the introduction of the outer condition. It likewise pieces infections and other hurtful substances.