In Mrs. Susna's class, 1/4 of her students are getting an A, 1/2 are getting a B, 1/8 are getting a C, 1/12 are getting a D, and 1/24 are getting a F. What fraction of Mrs. Susna's class getting a passing grade of C or higher.  Choose the correct option:
A) 5 / 6
B) 3 / 4
C) 7 / 8
D) 1 / 8
E) 1 / 2

Rationale: if mrs . susna ' s class has 24 students , the number of students receiving as , bs , or cs , are 6 , 12 , and 3 respectively . that means 21 / 24 of her students are receiving passing grades or 7 / 8 . ( answer : c )
Answer: C