Which is a complex sentence? A) After the massacre occurred, the government started an investigation, but no one expected to find anything
B) Although it will set you back at least a thousand dollars, a trip to Las Vegas is unforgettable

It’s not A, it’s B. A complex sentence contains an independent clause and a subordinate clause. In B, the independent clause (that can make sense on its own) is ‘a trip to Las Vegas is unforgettable’ and the subordinate clause (which can’t make sense on it’s own) is ‘although it will set you back at least a thousand dollars’. Hope this helps!

What are the examples