A recent survey was conducted to compare the cost of solar energy to the cost of gas or electric energy. Results of the survey revealed that the distribution of the amount of the monthly utility bill of a 3-bedroom house using gas or electric energy had a mean of $97 and a standard deviation of $12. If nothing is known about the shape of the distribution, what percentage of homes will have a monthly utility bill of less than $73?

a)at least 75%
b)at most 25%
c)at most 11.1%
d)at least 88.9%