In the 1920s, Edwin Hubble looked at the spectra of stars from distant galaxies. To his surprise, he found that all of these spectra were red-shifted, and that the more distant the galaxy, the greater the red-shift. What is a reasonable conclusion from this evidence?
A. The universe is expanding.
B. The universe is contracting.
C. The universe is unchanging.
D. The universe is curved.

A. The universe is expanding.

Explanation:

Let's begin by explaining what Hubble observed:

He observed several distant galaxies and obtained the spectrum of each one, after that he found out the spectral lines of most of these celestial bodies were displaced towards the red.  

In addition, many years before (1842) the Austrian physicist Christian Doppler proposed the existence of a particular effect that works for the waves:

In the case of electromagnetic waves, as the case of light in the stars and galaxies, there is a variation in the frequency (hence a variation in wavelength=color) due to the relative movement of the source of the wave with respect to its receiver.  

Later this was called the Doppler Effect.

Now, going back to Hubble; thanks to this he deduced the farther a galaxy is, the more redshifted it is in its spectrum.

This means that as a star moves away from the Earth (where the observer is), its espectrum turns towards the red.  And this is a strong and clear evidence of the expansion of the universe.

Therefore, the correct option is A.

it is A. The universe is expanding.

Explanation:

electromagnetic waves travel through space with vacuums. give me brainliest if right.

moving ball hits still ball. mover stops, hit ball moves. momentum transfer.