An blank should have only one independent variable

Mark's girlfriend dropped him off at a party, and then she went to visit her best friend two blocks away. when mark called his girlfriend to get a ride home, he could tell she had been drinking. what is mark's best plan of action? ride home with someone else and let his girlfriend drive herself get a friend to take him to his girlfriend's car so he can drive them both home ask his girlfriend to drive to the party, and then mark will drive them both home let his girlfriend drive them home slow to avoid getting pulled over