Scores on a University Exam Are Normally Distributed, with a Mean

Scores on a university exam are Normally distributed, with a mean of 78 and a standard deviation of 8. The professor teaching the class declares that a score of 70 or higher is required for a grade of at least C. Using the 68-95-99.7 rule, what percent of students failed to earn a grade of at least C?

A) 32%
B) 16%
C) 5%
D) 2.5%