The museum of science in boston has an exhibit in which metal balls drop down a chute, bounce around, and wind up in one of 21 bins. after 1000 balls have dropped, the heights within the bins clearly follow a bell-shaped curve. if the standard deviation is 3 bins, approximately how many balls are in bins 1 through 7?

a) 100
b) 160
c) 50
c) 330