However, the plesiosaur theory is not the only one. Many other ideas attempt to explain the sightings and "photographs." But two separate sonar surveys of the loch have not found any evidence of a creature larger than a salmon. The last survey,1 completed in 2007 by the British Broadcasting Company (BBC), involved 600 different sonar beams. Using satellite navigation systems, the team made sure every inch of the loch was searched. The team found nothing. What is the main idea of the bolded paragraph?
To explain the different theories surrounding the Loch Ness mystery
To explain the reasons people still believe that the Loch Ness Monster exists
To explain the ways people have tried to solve the Loch Ness Monster mystery
To explain why a plesiosaur cannot be Nessie