Which of the followings is not a multiple of $3$? 
Overseas Competition->Knowledge Point->Number Theory Modules->Factors and Multiples->Basic Concepts of Factors and Multiples
$$\text{A}$$. $$2+1+3=6=2\times3$$;  $$\text{B}$$. $$2+1+4=7$$, and $$7$$ is not a multiple of $$3$$;  $$\text{C}$$. $$2+1+6=9=3\times3$$;  $$\text{D}$$. $$2+1+9=12=4\times3$$.  We choose $$\text{B}$$. 