Given: (a - b)/(c - a) = 1
Multiply both sides by (c - a) to get: a - b = c - a
Add a to both sides to get: 2a - b = c
Add b to both sides to get: 2a = c + b
Divide both sides by 2 to get: a = (c + b)/2

Quantity A = the average of b and c
In other words, Quantity A = (b + c)/2
So, we have:
Quantity A: (b + c)/2
Quantity B: a

Well, from the given information, we learned that a = (c + b)/2

Answer: C

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