The following 25 test scores were recorded out of 100:
31,33,35,36,43,44,47,48,51,52,53,54,55,56,57,58,61,62,64,65,66,68,72,73,77
If the scores are grouped into the classes 30–39,40–49,50–59,60–69,70–79, what is the frequency of the class 50–59?
a) 7
b) 8
c) 6
d) 9