Calculate the future value of $ $1$ comma $000$ in

a$.4$ years at an interest rate of $6\%$ per year.

b$.8$ years at an interest rate of $6\%$ per year.

c$.4$ years at an interest rate of $12\%$ per year.

d$.$ Why is the amount of interest earned in part a less than half the amount of interest earned in part$$ b$?$