When the estimate of an assets useful life is changed,
a) depreciation exp for all past periods must be recalculated.
b) there is no change in the amount of depreciation exp recorded for future yrs
c) only the depreciation exp in the remaining yrs is changed
d) none of the above are true

What do you think the answer is?

Um I'm thinking u must have too much time on your hands, if your going to write, "what do you think the answer is?" how about getting a social life lol! Your too funny!

We are here to assist you, not do do your work for you. I will tell you that D is not the right answer and that when you change the useful life of an asset you have to recalculate depreciation for the remaining useful life of the asset.
So you should be able to figure your problem out, now.