Find the Present Value of the Future Value $$S ( t ) = S _ { 0 } e ^ { - a t }$$

Find the present value of the future value.
-The sales of a mature product (one which has passed its peak) will decline according to the function $$S ( t ) = S _ { 0 } e ^ { - a t }$$ , where t is time in years since the peak sales. Find the sales of a product 24 years after its peak sales if a = 0.23 and $$\mathrm { S } _ { 0 } = 35,000$$

A) 140 units
B) 10,996,672 units
C) 70 units
D) 44,051 units