With constant tax rates over time, why does a single-premium deferred annuity contract (SPDA) provide greater after-tax rates of return than does a money market account? How is the difference in...

With constant tax rates over time, why does a single-premium deferred annuity contract (SPDA) provide greater after-tax rates of return than does a money market account? How is the difference in after-tax accumulations in these two vehicles affected by the level of interest rates? Why does the length of the holding period affect the after-tax rates of return per period on SPDAs and not on money market accounts?