Among the depreciation methods allowed under U.S. GAAP(which one is correct)?

1.The rationale behind the accelerated depreciation method is that the asset's market value declines significantly in the early years of on its life.

2.In activity based methods service life is measured in terms of the total amount of the given activity expected to occur over the asset's useful life.

3.The most prevalent depreciation method is the activity based deprecation method.

4.The depreciable cost in the sum-of-years digit methods changes from year to year.