It is given that ξ = {x: x is a positive integer such that 3 ˂ x ≤ 18},
Y = {x: x is a multiple of 3}
Z = {x: x is a multiple of 9}.

(a) List all the elements in ξ, Y and Z in set notation.
(b) Draw a Venn diagram to represent the sets ξ, Y and Z.
(c) From the Venn diagram, find;
(i) Y ∪ Z
(ii) Y ∩ Z