The mean number of stereos sold per day at a store for 6 days was 13. Which statement must be true about the stereo sales at the store?

O The store sold 19 stereos for the 6 days
O The store sold 78 stereos for the 6 days
O The store sold 13 stereos on at least half of the 6 days
O The store sold 13 stereos on at least two of the days