If a number is written as $2a+4$, then $3$ times the number is~\uline{~~~~~~~~~~}~. (adapted from 1977 Math League.com contest problem, 8\textsuperscript{th}~Grade, Question \#31) 
选项为:A:$6a+4$  B:$2a+7$  C:$6a+12$  D:$32a+34$  请回答其中一个选项.