a shift toward longer wavelengths of light a shift toward shorter wavelengths of light the absence of wavelengths in the visible light range the compression of wavelengths in the visible light range
I gave my students this multiple choice question: Scientists measured the patterns of spectral lines given off by distant galaxies and concluded that almost all of these galaxies are moving away from Earth. What did the scientists observe directly?

Only one answer is correct among these 4 choices:
- a shift toward longer wavelengths of light
- a shift toward shorter wavelengths of light
- the absence of wavelengths in the visible light range
- the compression of wavelengths in the visible light range

Could you tell me which one is correct?

a shift toward longer wavelengths of light