Which is a complex sentence?

 - Unless the weather forecast is wrong, you'll probably need an umbrella tomorrow.
 - Babe Ruth hit 714 home runs during his baseball career but struck out 1,330 times.
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 unless.
Unless the weather forecast is wrong, you'll probably need an umbrella tomorrow.