(N/A) The cooling of hot water with time is governed by Newton's Law of Cooling,which states that the rate of loss of heat is proportional to the temperature difference between the body and its surroundings.
As time $t$ increases,the temperature $T$ of the hot water decreases.
The graph of temperature $T$ (on the $y$-axis) versus time $t$ (on the $x$-axis) is a downward-sloping curve that approaches the ambient temperature asymptotically.
Since the temperature $T$ decreases as time $t$ increases,the change in temperature $\Delta T$ is negative for a positive change in time $\Delta t$.
Therefore,the slope of the graph,given by $\frac{dT}{dt}$,is negative.