Unlike gradualism, punctuated equilibrium involves
a. a slow change in evolution over time.
b. stable periods of change interrupted by rapidly occurring change.
c. species that are said to be in equilibrium.
d. a steady change in evolution over time.

d. a steady change in evolution over time.

Unlike gradualism, is the process by which a certain specie or creature undergo changes and variations in a limited and solely focusedly confined in long period of time. 

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.

The equator absorbs the most sunlight