A bison is charging across the plain one morning. His path takes him to location $(x,y)$ at time $t$, where $x$ and $y$ are functions of $t$, and north is in the direction of increasing $y$. The temperature is always colder farther north. As time passes, the sun rises in the sky, sending out more heat, and a cold front blows in from the east. At time $t$ the air temperature $H$ near the bison is given by $H = f(x,y,t)$. The chain rule expresses the derivative $dH / dt$ as a sum of three terms:
$$\frac{dH}{dt} = \frac{\partial f}{\partial x} \frac{\partial x}{\partial t} + \frac{\partial f}{\partial y} \frac{\partial y}{\partial t} + \frac{\partial f}{\partial t}.$$
Identify the term that gives the contribution to the change in temperature experienced by the bison that is due to:

? A B C  1. The coming cold front ? A B C  2. The bison's change in latitude ? A B C  3. The rising sun

A. $\displaystyle \frac{\partial f}{\partial t}$ B. $\displaystyle \frac{\partial f}{\partial y} \frac{\partial y}{\partial t}$ C. $\displaystyle \frac{\partial f}{\partial x} \frac{\partial x}{\partial t}$