$$\text{A}$$: corresponding to $$\text{a}+\text{b}$$; $$\text{B}$$: corresponding to $$\text{a}-\text{b}$$; $$\text{C}$$: $$\text{na}$$, where $$\text{n}$$\emph{\emph{~}}is an integer. We choose $$\text{D}$$. 
Given that $$6$$ and $$9$$ are multiples of $$3$$, which of the followings might not be a multiple of $$3$$? 
[A] $$9 + 6$$ 
[B] $$9 - 6$$ 
[C] $$2 \times 9$$ 
[D] $$2 \times 9 + 1$$ 