15. Molly has a set of cards numbered 1 to 12 . She places eight of them at the vertices of an octagon so that the sum of every pair of numbers at opposite ends of an edge of the octagon is a multiple of 3 .
Which numbers did Molly not place?
A $1,5,9$ and 12
B $3,5,7$ and 9
C $1,2,11$ and 12
D 5,6,7 and 8
E $3,6,9$ and 12