Select the statement that is true concerning primary teeth.

there are 27 primary teeth, and the molars are permanent.
there are 32 primary teeth, and most children lose these teeth due to decay because they are never very strong.
there are 20 primary teeth, and by 24 months of age most children have all 20.
there are 24 primary teeth, and no new primary teeth appear after 13 months.

The correct answer is: There are 20 primary teeth, and by 24 months of age most children have all 20.

Primary teeth also known as deciduous teeth, milk teeth, are the first set of temporary teeth in the growth development of humans. They develop during the embryonic stage of development, become visible in the mouth during infancy (erupt) and usually get replaced by permanent teeth around age six (the first tooth). The primary teeth consist of 4 central incisors (one in each quadrant), 4 lateral incisors, 4 canines, 4 first molars, and 4 secondary molars.

the answer that best describes the relative refractory period in the neuron is the resting membrane potential is restored, but concentrations of sodium and potassium are not restored to their original state.

the refractory period is the time during which another stimulus given to the neuron will not lead to a second potential. thus, because na+ channels are inactivated during this time, additional depolarizing stimuli do not lead to new action potentials.  

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