Which best states the overall structure of walden?
a. the work describes the effect spending time in nature has on thoreau, and then explains why it has that effect.
b. the work describes thoreau’s thoughts over the course of a year spent immersed in the natural world.
c. the work compares thoreau’s thoughts before coming to walden to his thoughts afterward.
d. the work shows how various problems thoreau had before he came to walden are solved by his retreat.

a.) it provides a clue to solve the mystery.

the present continuous is a verb tense which is mainly used to indicates that an action is in progress at the moment of speaking.  

it is formed using: person + verb to be (am/is/are) +verb + - ing form  

example: i am studying at this moment .

in addition, the present continuous is also used in order to express things that will or will not happen in the near future.

example: i am having dinner with some friends tonight.

the speaker sometimes   uses it   to express that an action is repeated and it causes irritation on the part of the speaker

example: my sister is constantly complaining about everything.