Four numbers are written in a row. The average of the first two is $21$, the average of the middle two is $26$, and the average of the last two is $30$. What is the average of the first and last of the numbers? 
选项为:A:$$24$$  B:$$25$$  C:$$26$$  D:$$27$$  E:$$28$$  请回答其中一个选项.