Example of main idea

Unlike most other spider, the wolf spider spins no web. Instead it lives in a small hole in the ground. It lines this hole with silken material like that with which other spiders spin their web. Then the wolf spider makes a lid for its hole. The little creature even spins a silken hinge for the snug-fitting lid. Inside its burrow, this spider is safe from all enemies.

The paragraph tells mainly-
A. Why the wolf spider spins no web
B. How the wolf spider builds it's home
C. How the wolf spider spins it's web
D. Why the wolf spider lives in the ground