What value of $a$ would make the solution to the equation, $$ 2 a+3-4 x+7=3\left(-\frac{4}{3} x+7\right) $$,~"infinitely many solutions"?~\uline{~~~~~~~~~~}~ 
选项为:A:$-5$  B:$\frac{11}{2}$  C:$10.5$  D:$$21$$  请回答其中一个选项.