please help, i keep getting this question wrong

4. There was been a lot of discussion of airbome transmission of COVID-19. We can understand why this might be a problem by thinking about the physics that is involved. The virus can be contained in small spheres of water ("droplets"). We'll assume a droplet 20 um in diameter. The presence of the virus and other fluids doesn't change the density from that of pure water. (12 points) a. If we ignore any resistive forces, how long would it take such a droplet to fall to floor, starting from rest at a person's mouth (1.5m above the ground)? b. Now determine how long it takes to fall to the floor when starting from rest, but include the appropriate resistive force. You can make the same simplification we've used before, that the droplet very rapidly reaches terminal velocity. c. Based on your work in this question, is a reasonable to ignore the resistive force in considering the droplet's motion? Why or why not? d. What does your answer to part (b) say about the importance of air exchange and ventilation in enclosed spaces