Why is more than one reproductive isolation barrier needed to completely isolate a species?

This can be caused by mating differences, sterility or environmental barriers that eventually lead to the adaptive splitting into two species. However, reproductive isolation is not sufficient but internal barriers to gene flow are required for speciation to evolve.

option a is correct i.e.

explanation:

in context to the membrane of nerve impulse, the outer membrane of neuron is semi permeable that is only sodium ions can pass through the membrane i.e. smaller ions can pass through the membrane while larger ions cannot pass through it.