Why are humans most likely to form communities

Because humans want to help the world in their own way and want to help in a way that will promise to benefit the world

option a is correct i.e.

only sodium ions can pass through the membrane.

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.

Holozoic nutrition refers to the regular animal nutrition mode whereas saprophytic nutrition refers to the nutrition mode of fungi and bacteria.