What purpose do sentences 3 through 6 serve in this paragraph?

(1) my family has a quaint cottage at a small, private lake. (2) the cottage is about 75 years old, but we try to keep it in good condition. (3) the outside is white with green trim on the windows and shutters. (4) in front of the cottage is a flagstone walk that leads down to a stone patio. (5) the patio itself is right next to the water. (6) out from the patio stretches a dock where we keep a small rowboat tied. (7) every summer, my family sets aside a week when we all get together at the lake for a family vacation/reunion.

a. they set up a comparison.
b. they show cause and effect.
c. they provide supporting details.
d. they introduce the main idea.