$$\triangle$$ represents a number.$$\triangle$$代表一个数.  $$\triangle +\triangle =a, \triangle -\triangle =b, \triangle \times \triangle =c, \triangle \div \triangle =d,$$ and $$a+b+c+d=121$$. $$\triangle =$$~\uline{~~~~~~~~~~}~. 
选项为:A:$$8$$  B:$$9$$  C:$$10$$  D:$$11$$  E:$$12$$  请回答其中一个选项.