If a store: buy ten and get two free, * * * needs 120 balls, just buy 100 balls, and send another 20 balls, which needs 100x60=6000 yuan.

Store B: Cost 120x(60- 12)=5760 yuan.

If store C: buys 120x60 = 7200 yuan, and cashes back on 100/5 yuan, it will cash back 72x 15 =1080 yuan, so it will cost:

7200- 1080=6 120 yuan

By contrast, it is the cheapest in store B.