Questions

The long-run aggregate supply curve is vertical because it is at the full-employment or potential output. Because the economy's potential output is determined by the availability and productivity of real resources, not by the price level. That means that even if demand increases, firms can't hire new workers and expand because everyone is already working.

Since, the sentence starts talking about the thought process, R will be the next statement as it is talking about the two factors. Q will be the continuing statement as it carries a relative pronoun which connects both the statement. S follows Q as it discusses the need for a language and P discusses why there is a need for the international language.

The correct formation would be, ‘We can think of two factors which suggest the need for an international language often confused in the public mind’.