Choose the correct a lternative to fill in the blanks respectively.

The source of the oil spill washing up on thousands of kilometres of Brazil's coastline _______ the past few weeks remains the subject of a great deal of speculation. We've been asking experts on shipping routes and the types of crude oil produced in different countries to try to identify the _______ likely source of the spill.First detected in early September, crude oil has continued to appear on the beaches and pristine mangroves of north-east Brazil.Nine states have been cleaning thick sludge from their _______. More than 1,000 tonnes of oil have been removed so far. Most of the investigations so far have been _______ by Brazilian organisations. The Federal University of Bahia took samples of oil from beaches in two states. Source:

https://www.bbc.com/news/world-latin-america-50223106