Why are the two strands of the double helix "perfect and specific compliments"?

Because for the strands to form a double helix the base pairs have to match perfectly (adenine with thymine and cytosine with guanine) in DNA , and if a strand of RNA is paired with a split strand of DNA it goes adenine with uracil and cytosine with guanine (uracil replaces thymine)

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 answer would be sand and gravel