【单选题】
数据70,71,72,73的标准差是( )
①
<img class="kfformula" src="20190809/1565330298796439.png" data-latex="\sqrt {2}"/>
②
<img class="kfformula" src="20190809/1565330298960506.png" data-latex="\frac {\sqrt {5}} {2}" width="77" height="66"/>
③
<img class="kfformula" src="20190809/1565330298946541.png" data-latex="2"/>
④
<img class="kfformula" src="20190809/1565330298247161.png" data-latex="\frac {5} {3}" width="84" height="70"/>