The mean, median, and unique mode of the positive integers $3,4,5,6,6,7$, and $x$ are all equal. What is the value of $x$ ? (2012 AMC8, Question \#11) 
选项为:A:$$5$$  B:$$6$$  C:$$7$$  D:$$11$$  E:$$12$$  请回答其中一个选项.