Nicotinic receptors a. are found at neuroeffector junctions of the parasympathetic nervous system. b. respond to epinephrine. c. open chemically-gated sodium ion channels. d. respond to norepinephrine. e. are found at neuroeffector junctions of the sympathetic nervous system.

Option C, open chemically gated sodium ion channels.

Explanation:

The nicotinic receptors are present on the muscle cell membrane. These receptors belong to the class of cholinergic receptor which binds to the acetylcholine and opens the gated channel to allow transfusion of cation through it. These actions of the receptors are stimulated by the neurotransmitter. Nicotinic receptors are very fast at acting and hence are more prevalent at the neuromuscular junction.

Hence, option  C is correct

answer: ice crystals in air

the water from the water body or another source of water gets evaporated as water vapor as an effect of heat on water. these water vapor rises in the upper layers of the earth atmosphere under high atmospheric pressure. the atmospheric pressure of these water molecules starts to decrease due to the effect of cold air. the invisible water vapor condenses into visible water droplets and ice crystals.these ice crystals and water droplets being less denser than the surrounding air floats on air as clouds.