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# If A={3,6,9,12,15,18,21},B={4,8,12,16,20},C={2,4,6,8,10,12,14,16},D={5,10,15,20} (ii) A-C

$\begin{array}{1 1}(A)\;\{3,9,15,18,21\}&(B)\;\{3,6,9,12,15,18,21\}\\(C)\;\{2,4,6,8,10,12,14,16\}&(D)\;\phi\end{array}$

$A=\{3,6,9,12,15,18,21\}$
$C=\{2,4,6,8,10,12,14,16\}$
Here 6,12 are the only elements of set A are common to set C
$\therefore$ Difference of sets
$\Rightarrow A-C=\{3,6,9,12,15,18,21\}-\{2,4,6,8,10,12,14,16\}$
$\Rightarrow \{3,9,15,18,21\}$
Hence (A) is the correct answer.