BRUSSELS (Reuters) - After nearly five years of Brexit crisis, the European Union and Britain are making a last ditch attempt to clinch a thin trade deal that would govern nearly a trillion dollars in annual imports and exports from 2021.The United Kingdom left the EU last January but the trade deal would kick in when it leaves informal membership - known as the transition period - in nine weeks time. If a deal can be done before the transition period ends on Dec. 31, the two sides would sign more than 1,000 pages of international treaties covering everything from smoked salmon and cheese to car parts and medicine. So far, there is no breakthrough, though talks in London and Brussels made progress on unifying texts each side has so far prepared separately. Sticking points remain on economic fair play, fisheries or settling disputes, sources on both sides said. Here are three main scenarios for Brexit.