Suppose that the cost of drilling $\small {x}$ feet for an oil well is $\small{C = f(x)}$ dollars.

(a) What are the units of $\small{f\;'(x)}$? (Write each word in its entirety, in lower case).

Units: /

(b) If you've already drilled 590 feet, estimate the cost of drilling an additional foot if $\small{f\;'(590) = 700}$.

Cost: