On January 1, 2019,the MA Company purchased a machine for $36,000 that had a ten-year estimated useful life and no salvage value.At the start of the seventh year,a new energy saving device was added to the machine that extended its original useful life by two years.This change should be accounted for in the seventh year by

a.depreciating the remaining book value over four years.

b.retroactively adjusting income of prior period using the adjusted useful life.

c.including the cumulative effect of the change in net income for the current period.

d.depreciating the remaining book value over six years.