Which equation is an identity?

3w + 8 – w = 4w – 2(w – 4)

8 – (6v + 7) = –6v – 1

5y + 5 = 5y – 6

6m – 6 = 7m + 9 – m

Real gross domestic product: a) will increase if the price level increases. b) can change from one year to the next even if there is no change in output. c) will increase if the level of output increases. d) is a measure of inflation.