It takes Mary $$30$$ minutes to walk uphill $$300 $$ m from her home to school, but it takes her only $$10$$ minutes to walk from school to home along the same route. What is her average speed, in m/min, for the round trip? (adapted from 2003 AMC 10 Problem, Question \#4) 
选项为:A:$$10$$  B:$$30$$  C:$$20$$  D:$$15$$  E:$$12$$  请回答其中一个选项.