If the average (arithmetic mean) of a and b is 50 and the average of b and c is 70, what is the value of c − a?	Pick one.
A) 40	B) 50	C) 90	D) 140	E) it can not be determined from the information given .

Rationale: "- ( a + b = 100 ) b + c = 140 c - a = 40 a . 40"
Answer: A