It's 10 points, not 20. also, this isn't how Brainly should be used. Brainly isn't a "do my homework for me" website, it's a "help me with my homework" website. What you should do is research and write on your own, and ask for help/guidance with it on here
The usa fought in vietnam because southern vietnam was not communist while the north were pushing for communism so the answer would be d to stop the spread of communism