Burian is a very prolific writer, able to write thousands of pages per day! His assistant identified a few relevant functional relationships and modeled them: Function | Input | Output - | - | - $P$ | Amount of sugar in Burian's morning coffee, $x$ | Number of pages Burian writes, $P(x)$ $I$ | Number of pages Burian writes, $k$ | Number of ink cartridges used, $I(k)$ $S$| Number of ink cartridges used, $r$ | Number of times the assistant has to walk to the office supply shop, $S(r)$ **Which of the following expressions represents the number of ink cartridges used as a function of the amount of sugar in Burian's morning coffee?** [[☃ radio 1]]