Avenn diagram is shown below:

what are the elements of (a ∩ b) ' ?

a. {3,4,5,6}
b. {1,2,5,6}
c. {1,2,3,4}
d. {1,2,3,4,5}

i was under the assumption that the intersection of 2 sets was the elements present in both sets so i thought the answer was just {1,2} but apparently not. anyone who can answer explain where my mistake was.