A meteorological model gives the ground temperature (in ∘C) at a desert site as T(r)=25−2r, where r (in kilometers) is the distance from a heat source. A hiker moves away from the source along a straight path so that r(t)=3t+4, where t is the time in hours since noon.
Determine dtdT at t=2 hours. (Units: ∘C/h)
a) 6
b) −3
c) −6
d) −2