In my opinion, the correct option is <span>2 ("There is substantial uncertainty around by how much the sea levels may actually increase," said Professor David Vaughan from the British Antarctic Survey during his presentation. "Once sea level rises, it is quite difficult to make it fall again. At the moment, the sea level is increasing by about 3 mm per year, but with climate change, this rate is very likely to increase.") Obviously, Professor David Vaughan is an expert on this matter. The excerpt No. 3 also mentions him, but it doesn't tell us who he is, so we can't assess his expertise from it. The excerpt No. 6 cites another person, but he is just chair of an organization - we don't see if he is really an expert or not.</span>
B."There is substantial uncertainty around by how much the sea levels may actually increase," said Professor David Vaughan from the British Antarctic Survey during his presentation. "Once sea level rises, it is quite difficult to make it fall again. At the moment, the sea level is increasing by about 3 mm per year, but with climate change, this rate is very likely to increase."
