A steel casting cools to 90 percent of the original temperature difference in 30 min in still air. The time it takes to cool this same casting to 90 percent of the original temperature difference in a moving air stream whose convective heat transfer coefficient is 5 times that of still air is
(a) 3 min

(b) 6 min

(c) 9 min

(d) 12 min

(e) 15 min