【单选题】
下列函数中是奇函数的是( )
①
<img class="kfformula" src="20190808/1565253233633468.png" data-latex="y=x+{x}^{2}"/>
②
<img class="kfformula" src="20190808/1565253233837122.png" data-latex="y=-\sqrt {x}"/>
③
<img class="kfformula" src="20190808/1565253233536783.png" data-latex="y=\frac {x} {1-{x}^{2}}"/>
④
<img class="kfformula" src="20190808/1565253233611630.png" data-latex="y=5{x}^{2}"/>