Answer
a. No, because $C(90)$ gives the number of bushels of corn which can only, at lowest, be 0.
b. Bushels of Corn
c. Yes because $\frac{dw}{dt}|_{f=90}$ is the number of bushels per week and it can be negative if it is decreasing.
Work Step by Step
a. We know that $C(90)$ cannot be negative because you cannot have less that 0 bushels of corn.
b. We can see that the units are bushels of corn by looking at the question.
c. We can see that $\frac{dw}{dt}|_{f=90}$ can be negative because the number of bushels per week can be decreasing from the last week.