Researchers continue to study the effectiveness of vaccines against COVID-19. Recently a study was completed comparing 352,878 adults who had received 2 doses of the Moderna vaccine to 352,878 adults who had not been vaccinated. Of those who had been vaccinated, 289 had been diagnosed with COVID-19 within a 3-month period, while 1,144 of those who had not been vaccinated were diagnosed in that same period. This gave a mean cases-per-1000 of 2.77 for the vaccinated group and a mean cases-per-1000 of 20.2 for the unvaccinated group. We are interested in estimating the difference in mean cases-per-1000 between vaccinated and unvaccinated individuals. The standard error for this estimate is 0.75. For a) and b), use one of the following options 1 2 1 2 1 2 1 2 a. (2 pts) Choose the option above that gives notation for the quantity being estimated in this study. b. (2 pts) Choose the option above that gives a notation for the quantity that gives the best estimate.