The following box plot shows the number of years during which 20 schools have participated in an interschool swimming meet: A box and whisker plot is drawn using a number line from 0 to 10 with primary markings and labels at 0, 5, 10. In between two primary markings are 4 secondary markings. The box extends from 1 to 6 on the number line. There is a vertical line at 3.5. The whiskers end at 0 and 8. Above the plot is written Duration of Participation. Below the plot is written Years.

At least how many schools have participated for 6 years or more? (5 points)

a
4 schools

b
5 schools

c
10 schools

d
11 schools

Please explain your answer