Given two basic functions that model a real-world situation, compose them in order to model a more complex situation.

Problem

Mason is a captain of a tourist cruise to the Statue of Liberty. He identified a few relevant functional relationships and modeled them:
FunctionInputOutput
PDuration of the cruise, rAverage number of photos taken by each passenger, P, left parenthesis, r, right parenthesis
DSpeed of the ship, kDuration of the cruise, D, left parenthesis, k, right parenthesis
SRate of the river current, xSpeed of the ship, S, left parenthesis, x, right parenthesis
The expression D, left parenthesis, S, left parenthesis, x, right parenthesis, right parenthesis represents which of the following?
Please choose from one of the following options.