A+  Super Answer with new insight. 
A   Excellent Answer.  Generally correct. 
A-  Norm.  Genuine effort with not too many or too fundamental mistakes. 
B+  Genuine effort but with too many or too fundamental mistakes. 
B   Below norm 
B-  Much below norm 
C   Disastrous 

A+  4.33
A   4.00
A-  3.67
B+  3.33
B   3.00
B-  2.67
C   2.00