Which is a complex sentence?
A: Because the leaves blocked the intense sun, it was cool underneath the forest canopy.
B: Her dog happily fetched his tennis ball from the muddy puddle and eagerly waited for her to throw it again.
The first sentence is the complex sentence. It is made up of an independent clause and a dependent clause. The dependent clause begins with the subordinating conjunction because.
Because the leaves blocked the intense sun, it was cool underneath the forest canopy.