Let a , b and c be three non-zero vectors, no two of which are collinear. If the vector a + 2 b is collinear with c , and b + 3 c is collinear with a , then a + 2 b + 6 c is equal to ( λ being some non-zero scalar) then a + 2 b + 6 c =
a) λa
b) λb
c) λc
d) 0