Why would sound get more intense at night?
a. Low temperature. We can rule this out, because temperature's effect on sound conductiving is related through a consequence of the temperature change.
b. Increase in density. How is sound conducted through the air? It is conducted by waves of air getting closer together and further apart. This movement comes from air molecules bouncing into each other. Would increasing density better facilitate this? Yes.
And what happens to the air at night as it cools? It grows more dense.
c. Decrease in density. Does the air grow less dense as it cools? We just identified that the opposite occurs, so this is out.
d. Calmness. While calmness can better facilitate the transmission of sound, and reduce the likelihood of destructive interference, this wouldn't change the intensity of the sound, just our perception of it. And Calmness isn't a constant at night, some places don't get wind until the sun goes down.
The correct answer is B. The increasing density of the air, due to the cooling, enables air molecules to bump into each other more readily, better facilitating the transmission of sound waves, making sounds more intense at night.