Which sentence is a run-on? (i feel like this should be easy, but i learned this in 5th, so i have no idea what i'm

a) kathy and jerry worked very hard on their project; therefore, they felt they deserved a high grade.

b) the small girl seemed to be lost, she was crying and looking for her mother.

c) we are moving to cincinnati in july, and i'll go to school there.

d) billy and chris ran down the court and blocked the shot of the opposing player.