Answer
B. 20 K
Work Step by Step
We are able to find $\Delta T$ in degree Celsius:
$$\Delta T = 60^{\circ} C - 40^{\circ}C = 20^{\circ}C$$
We cannot convert 20$^{\circ}C$ into kelvins by adding 273 in this case because $\Delta T$ is not a mere scalar quantity. $\Delta T$ is the difference between two quantities, so we have to take into account the unit size of the Celsius scale and the Kelvin scale. Since their unit size is equal (for example, water's boiling and freezing point are both 100$^{\circ}C$ and 100$^{\circ}K$ apart), $\Delta T$ written in kelvins is 20$^{\circ}K$.
