【单选题】
a?a=5,则∣a∣=( )
①
<img class="kfformula" src="20190809/1565319639836572.png" data-latex="5"/>
②
<img class="kfformula" src="20190809/1565319639359617.png" data-latex="\sqrt {5}"/>
③
<img class="kfformula" src="20190809/1565319639607345.png" data-latex="25"/>
④
<img class="kfformula" src="20190809/1565319639200781.png" data-latex="1"/>