A man was stabbed on a CTA bus Wednesday morning in Greater Grand Crossing on the South Side, authorities said. About 9:30 a.m., two people on a westbound No. 79 bus were arguing while in the first block of West 79th Street when one of them pulled out a sharp object and cut the other person, according to Chicago police and the CTA.The victim, 22, suffered a laceration to his lower back and was taken to Christ Medical Center in Oak Lawn, where he was listed in serious condition, police said.No one is in custody, and detectives are investigating.