Young Corporation decided to change its depreciation policy by (1) changing from double-declining-balance depreciation, and (2) changing the estimated useful life on all automobiles used in the business from five years to four years. Which of the following is correct concerning these two changes?

a. both are changes in accounting principles

b. one is an error correction, and one is a change in accounting principle.

c. one is a change in estimate effected through a change in accounting principle and one is a change in estimate.