A = excellent
B = good
C = fair, a solid pass
D = poor, just barely passing
F = failing, does not meet minimum standards
There are no predefined percentage cut-offs for the various grades.
The translation of percentages to letter grades will be tailored
depending on the difficulty of the exam,
but not a function of the class performance on the exam.
There is not a predefined fraction of the class that will
get any particular grade on any exam or overall.
The law of large numbers often applies, and the class average
tends to be between C and B, but cohorts do vary in performance.