If the average (arithmetic mean) of a and b is 30 and the average of b and c is 60, what is the value of c − a?  Choose the correct choice from the following answers.
A) 25
B) 60
C) 90
D) 140
E) it can not be determined from the information given .

Rationale: - ( a + b = 60 ) b + c = 120 c - a = 60 b . 60
Answer: B