Amy mixes $$10$$ g of a $$20\textbackslash\%$$ sugar solution and $$40$$ g of a $$25\textbackslash\%$$ sugar solution together. After~\uline{~~~~~~~~~~}~of water evaporates, the percent concentration of solution is $40\textbackslash\%$. 
选项为:A:$$15$$ g  B:$$18$$ g  C:$$20$$ g  D:$$25$$ g  请回答其中一个选项.