Circle the answer that correctly completes each sentence. 1. The Incas didn’t have any sophisticated machines or work animals. The temples ( must / couldn’t / may ) have been built exclusively by human hands. 2. Dinosaurs may not have died off gradually as the result of climate change. They ( had to / could / would ) have been killed off quickly by some sudden event. 3. I don’t believe in any of those other crazy theories. Stonehenge ( might have been / were / had to have been ) built by ancient Britons many years ago. 4. The Nazca lines are incredible, but I’m certain they weren’t created by people who lived 1,500 years ago. The ancient people ( couldn’t have / would have / must have ) had the knowledge to create something so accurate. 5. Conspiracy fans continue to consider the story of Roswell aliens. They believe that UFOs ( might / couldn’t / had to ) have crashed in the desert. 6. The Mary Celeste ( might have been / had to have been / had been ) attacked by pirates. No one knows for sure. 7. It’s obvious that something terrible happened. The crew ( had to have / would have / might ) abandoned the ship during the incident. There’s no other explanation.