A company's manufacturing process uses 500 gallons of water at a time. A "scrubbing" machine then removes most of a chemical pollutant before pumping the water into a nearby lake. To meet federal gregulations the treated water must not contain more than 80 parts per million (ppm) of the chemical. The machine's output can be described by a normal model with standard deviation 4.2 ppm. The company's lawyers insist that no more than 2% of the treated water be over the limit. To achieve this, to what mean should the company se the scrubbing machine
a. 80 ppm
b. 69.459 ppm
c. 88.626 ppm
d. 75 ppm
e. 71.374 ppm