Irene mixes $$100$$ kilograms of dogfood that contains $$50\textbackslash\%$$ rice with $$400$$ kilograms of dogfood that contains $$80\textbackslash\%$$ rice. Find the percent concentration of the rice in the new mixture. 
选项为:A:$$70\textbackslash\%$$  B:$$72\textbackslash\%$$  C:$$75\textbackslash\%$$  D:$$74\textbackslash\%$$  请回答其中一个选项.