You have been given a task to install grass. The area of the lawn is 30 square meters, and it will take 5 more days for the supplier to deliver the required quantity of grass. At your best performance, you usually take 10 days to complete the task and in the worst case, it might take 22 days. However, most of the time you take around 15 days to complete the task, assuming no unfavorable events happen. What is the standard deviation for this activity?

(a) 4

(b) 2

(c) 16

(d) 15