There is a gasoline spill in a major river. The mayor of a large downstream city demands an estimate of how many hours it will take for the spill to get to the water supply plant intake. The emergency responders measure the speed of the leading edge of the spill, effectively focusing on one particle of fluid. Meanwhile, environmental engineers at the local university employ a computer model, which simulates the velocity field for any stage of the river and for all locations (including steep narrow canyon sections with fast velocities and an extremely wide reach with slow velocities). To compare these two mathematical approaches, which of the following statements is most correct?

a. Th e responders have an Eulerian approach, and the engineers have a Lagrangian one.

b. Th e responders have a Lagrangian approach, and the engineers have an Eulerian one.