a 2020 study by the royal society finds that global sea level is rising at an average rate of 3.4 mm/yr. according to syvitski et al. (2009), relative sea level on the pearl river delta is rising at 7.5 mm/yr. based on these results, what is the rate of land subsidence (mm/yr) in the pearl river delta? round your answer to the nearest tenth of a mm/yr