If $f(x)$ is the slope of a trail at a distance $x$ miles from the start of the trail, what does $\int_{3}^{\,5} {f(x)}\,dx$ represent?

A. The length of the first 3 miles of the trail.
B. The length of the trail between $x = 3$ mi and $x = 5$ mi from the start of the trail.
C. The change in elevation over the first three miles of the trail.
D. The change in elevation between $x = 3$ mi and $x = 5$ mi from the start of the trail.