according to this excerpt from walden, in which subject area did the transcendentalist thinker thoreau most likely find his inspiration?

this small lake was of most value as a neighbor in the intervals of a gentle rain-storm in august, when, both air and water being perfectly still, but the sky overcast, mid-afternoon had all the serenity of evening, and the wood thrush sang around, and was heard from shore to shore. a lake like this is never smoother than at such a time; and the clear portion of the air above it being, shallow and darkened by clouds, the water, full of light and reflections, becomes a lower heaven itself so much the more important. from a hill-top near by, where the wood had been recently cut off, there was a pleasing vista southward across the pond, through a wide indentation in the hills which form the shore there, where their opposite sides sloping toward each other suggested a stream flowing out in that direction through a wooded valley, but stream there was none. that way i looked between and over the near green hills to some distant and higher ones in the horizon, tinged with blue.

a.
human behavior
b.
nature
c.
civil war
d.
history
e.
nonconformity