A few causes contributed to my treehouse collapsing. It wasn’t waterproofed, and it was hit by several heavy rainstorms. The damp wood began to rot. Whenever we climbed into the treehouse, you could feel some of the nails pulling free from the branches. One night, the treehouse fell out of the tree. Which is the topic sentence?

a | Whenever we climbed into the treehouse, you could feel some of the nails pulling free from the branches.
b| One night, the treehouse fell out of the tree.
c| A few causes contributed to my treehouse collapsing
d | The damp wood began to rot.