f$.$ Why does it take more per month when she is putting money away at $8\%$ than when she was earning a lower rate of $5\%$ over the $51$ years?

A$.$ It is because the investment period is shorter even though the interest rate is high enough.

B$.$ It is because the investment period is shorter and the interest rate, though higher, is not high enough.

C$.$ It is because the interest rate is higher regardless of the investment period.

D$.$ It is because the investment period is shorter regardless of the interest rate.