$35=5\times 7$  $84=2\times 2\times 3\times 7$  Because $B$ is the factor both number contains, $B=7$  Thus, $A=5$, $C=12$. 
There are three whole number $A$, $B$, $C$. $A\times B=35$, $B\times C=84$. $A+B+C=$~\uline{~~~~~~~~~~}~. 

A. $$20$$ 
B. $$23$$ 
C. $$24$$ 
D. $$29$$ 