Which of the following is true about the speed of light?
a - it is a constant when the light is traveling in a vacuum.
b - it speeds up or slows down depending on the observer.
c - it is slowest in a vacuum.
d - it varies depending on the color of the light.

A - It is a constant when the light is traveling in a vacuum.

Explanation: "Light in a vacuum always travels at the same speed." Those additional three words in a vacuum are very important. A vacuum is a region with no matter in it. So a vacuum would not contain any dust particles (unlike a vacuum cleaner, which is generally full of dust particles).

The answer is A. It is a constant when the light is traveling in a vacuum.

c) up

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