Which is a complex sentence?
A: If you thought that dish was spicy, this dish will set your mouth on fire.
B: Avalanches can reach speeds of eighty miles per hour within five seconds.
The second 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 if.
If you thought that dish was spicy, this dish will set your mouth on fire.