Based on the American Chemical Society, there is a 0.9 probability that in the United States, a randomly selected dollar bill is tainted with traces of cocaine. Assume that eight dollar bills are randomly selected.
a. Find the probability that all of them have traces of cocaine.
b. Find the probability that exactly seven of them have traces of cocaine.
c. Find the probability that the number of dollar bills with traces of cocaine is seven or more.
d. If we randomly select eight dollar bills, is seven an unusually high number for those with traces of cocaine?