If $A=\{1,2,3\}$ and $B=\{2,3,4,9\}$, then which of the following is a function $f:A \;\rightarrow \;B$ ?

Question

(A) $f(x)=x+1$

(B) $f(x)=2x$

(C) $f(x)=\sqrt x$

(D) $f(x)=x$