On January 1, year 2, an intangible asset with a 35 year estimated useful life was acquired. On January 1, year 6, a review was made of the asset and it was determined that the estimated useful life was 45 more years. As a result of the review:

a. The unamortized cost should be amortized over a 45 year life

b. The unamortized cost should be amortized over a 40 year life

c. The original cost should be amortized over a 50 year life

d. The original cost should be amortized over a 39 year life

Explain the reason.