What causes stars to twinkle?
a. bending of light rays by turbulent layers in the atmosphere variable absorption by interstellar gas along the line of sight to the star
b. it is intrinsic to the stars–their brightness varies as they expand and contract.
c. variations in the absorption of the atmosphere the inability of the human eye to see faint objects

a. bending of light rays by turbulent layers in the atmosphere, variable absorption by interstellar gas along the line of sight to the star.

Explanation:

Light is refracted as it passes from one media to another, that same scenario is what causes that stars twinkle. When the light coming from a distant star reaches the Earth atmosphere, the beams of light will be refracted over and over as a consequence of the different temperature and density on the layers of the atmosphere, getting as result a zigzag pattern (twinkle).

Another cause of twinkle is the interstellar gas. The light will be absorbed in a variable way along the line of sight between the star and the Earth by different lumps in the insterstellar gas.      

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explanation: