To find the amount of substance left after 18 days using the function \( A(t) = 135(0.917)^t \), we will substitute \( t = 18 \) into the function.
Let's calculate \( A(18) \):
\[ A(18) = 135(0.917)^{18} \]
First, calculate \( (0.917)^{18} \):
\[ (0.917)^{18} \approx 0.292 \]
Then, multiply that result by 135:
\[ A(18) \approx 135 \times 0.292 \approx 39.42 \]
Now rounding this to the nearest whole number gives us:
\[ A(18) \approx 39 \]
Since the nearest whole number is not given in your responses, it seems like you may need to verify the calculations or see if there was a misunderstanding with the function. The correct value appears to be around 39, which isn't listed among the provided responses. Please ensure the function or responses are correctly stated.