What are so tiny that they are measured at only a few thousand light-years or less and astronomers think that there may be billions of them in our universe?

The correct answer is STARS

Explanation:

Billions of stars are believed to be in different galaxies and a lot more billions are believed to be in our universe. These stars are small/tiny that they are a few thousand light-years away. Light-year is the unit for measuring distance of bodies in space which can be described as how far a beam of light travels in a year.

the most speciallized is b. grizzly bear

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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