The Environmental Protection Agency (EPA) has established national ambient air quality standards in an effort to control air pollution. Currently, the EPA limit on ozone levels in air is 12 parts per hundred million (pphm). A study examined the long-term trend in daily ozone levels in Houston, Texas. One of the variables of interest is Y, the number of days in a year on which the ozone level exceeds the EPA 12 pphm threshold. The mean number of exceedances in a year is estimated to be 18. Assume appropriate probability distribution for Y and provide logical reasoning behind your assumption.