Which of these data sets is symmetrical?

a. 1, 7, 13, 13, 13, 19, 19, 19, 25, 31, 37
b. 1, 1, 13, 13, 19, 19, 19, 25, 25, 31, 37
c. 1, 7, 13, 13, 19, 19, 19, 25, 25, 31, 37
d. 1, 7, 13, 19, 19, 19, 25, 25, 25, 31, 37