One way to make a letter grade is:
One A, 11 B's and a C.
The curve is B centered because question 1 is essential,
we can't call our work exceptional until we fix our understanding
of question 1.
Here is the grade distribution:

25
24 X
23
22
21
20 XXXXX
19 XXXX
18 X   
17 X
16
.
.
.  X
.