Which statement is the best interpretation of the number 1.8 in this context?

The given function, `D(t) = 850 - 1.8t`, is a linear model of the form `y = b + mx`, where `y` is the dependent variable (`D`), `x` is the independent variable (`t`), `b` is the y-intercept, and `m` is the slope. In this context, the slope `m` represents the rate of change of `D` with respect to `t`.

Here, the slope is -1.8. The variable `D` is in megabytes and the variable `t` is in seconds, so the rate of change is in megabytes per second. The negative sign indicates that the amount of data remaining is decreasing.

Therefore, the number 1.8 represents the rate at which the file size is decreasing, in megabytes per second.

- · *A is incorrect. · * This choice mistakes the rate for the total time. The total time to complete the download would be the value of `t` when `D(t) = 0`, which is `0 = 850 - 1.8t`, or `t = 850 / 1.8 ≈ 472` seconds.

- · *B is incorrect. · * This choice mistakes the rate (slope) for the initial value (y-intercept). The initial size of the file is the value of `D` at `t=0`, which is `D(0) = 850 - 1.8(0) = 850` megabytes.

- · *C is correct. · * It correctly identifies 1.8 as the rate of decrease in megabytes per second.

- · *D is incorrect. · * This choice describes the inverse of the rate. A rate of "1.8 seconds per megabyte" would be `t/D`, but the model's rate is `D/t`, which is 1.8 megabytes per second.