Which type of joint allows the rounded end of one bone to fit into a ring of bone so the the bone can turn
a. plane
b. pivot
c. condyloid
d. saddle

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.

i think the answer is b.