Which sentence is a run-on sentence?
A.
Michael needed more paper for the printer, so he bought some when he went to the office supply store.
B.
Stephanie ran out of detergent, she made a note to buy some more the next time she went to the store.
C.
Whenever Michael went to the library, he checked out at least three books to read.
D.
Andrea knew she was ready to run in the marathon; she had trained for weeks prior to the race.