We can detect 21-cm emission from other galaxies as well as from our own Galaxy. However, 21-cm emission from our own Galaxy fills most of the sky, so we usually see both at once. How can we distinguish the extragalactic 21-cm emission from that arising in our own Galaxy

the relativistic Doppler effect,

Explanation:

The emission of 21 cm from hydrogen atoms suffers from the relativistic Doppler effect, this is a current of its wavelength due to the relative speed between the source and the observer, us located on Earth.

If the object is moving away from us the shift is towards red and if the body is approaching towards us the shift is towards blue.

Also the value of the shift depends on the speed of the body.

with the invention of the automobile, the woman of the house could drive to the store and get groceries. women were also free to socialize more. during world war ii women were allowed to work in factories to the war effort and could drive to work or ride mass transit buses to their jobs.

i think its c if i'm wrong