The sales in thousands of a new type of product are given by $S(t)=70-60 e^{-0.1 t}$, where $t$ represents time in years. Find the rate of change of sales at the time when $t=4$, Round to the nearest tenth. A. -4 thousand per year B. 4.0 thousand per year C. -9 thousand per year D. 9.0 thousand per year

Step 1 ：Given the sales function \(S(t)=70-60 e^{-0.1 t}\), we need to find the rate of change of sales at the time when \(t=4\).

Step 2 ：The rate of change of sales at a given time is given by the derivative of the sales function with respect to time.

Step 3 ：Find the derivative of the function \(S(t)=70-60 e^{-0.1 t}\), which is \(S'(t) = 6.0 e^{-0.1 t}\).

Step 4 ：Substitute \(t=4\) into the derivative function to get the rate of change at \(t=4\), which is approximately 4.02.

Step 5 ：Round the result to the nearest tenth to get 4.0.

Step 6 ：\(\boxed{4.0 \text{ thousand per year}}\) is the rate of change of sales at the time when \(t=4\).