another time mgmt ques

 

 

  Activity 1 has a duration of 20 days, activity 2 of 10 days, activity 3 of 5 days and activity 4 of 6 days.

What is the minimum total duration between the milestones A and B?

  36 days
  37 days
  39 days
  42 days

My answer was A but right answer is A

Can you please explain

 see old posts