Which of the following sets is a subset of a = { 8 , 9 , 2 , 444 , 90 , 67 , 8 }?

a. t = { 8 , 9 , 99 , 2 }
b. q = { 444 , 99 , 8 }
c. r = { 8 , 444 , 90 , 8 }
d. s = { 2 , 444 , 90 , 5 }