A run-on is twosimple sentences incorrectly joined with no coordinating conjunction and nocomma.
Subject + verb + subject + verb
Example: 
Franciscoworked out Mariacleaned her room. X

The first test was difficult thesecond one was easy. X

Whichis the best way to rewrite the following sentence?
They don’t buy junkfood because they like healthy food.
A.They don’t buy junk food, so they like healthy food.
B.They don’t buy junk food, but they like healthy food.
C.They like healthy food, so they don’t buy junk food.
D.They like healthy food, but they don’t buy junk food.