Why does the Doppler effect detect only radial velocity?

Question

Why does the Doppler effect detect only radial velocity?

a. Only objects moving directly toward or away from the observer can emit waves that can be detected.

b. For objects moving perpendicular to the line of sight, the wavelength shift can be measured only with a great error.

c. In space all bodies either converge or retreat, therefore they have only radial velocity.

d. Motion of a wave source perpendicular to the line of sight cannot cause a wavelength shift because such motion doesn’t make peaks of waves closer together or farther apart.

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