Although having evolved independently, the tracheal tubes of mammals and insects are both supported by rigid tissues. the trachea of a mammal is supported by cartilage, and the tracheae of an insect are supported by chitin. what selective pressure most likely led to the convergent evolution of these respiratory structures?

The answer is B.  Which is “When the air is in the respiratory medium, there is a greater risk that the tracheal tubes will collapse. The cartilage in the respiratory system gives support to the tubes against negative or positive pressure, during respiration, hence keep it open.  

not sure if this what   you'r looking for would be percent composition