当时，下列变量为无穷小量的是（　）.

<img src、"http://oss.ouchn.cn/ddzx/N601/gongxue/jjsx/img/1-3/image018.png" alt="" align="middle">

<img src、"http://oss.ouchn.cn/ddzx/N601/gongxue/jjsx/img/1-3/image019.png" alt="" align="middle">

<img src、"http://oss.ouchn.cn/ddzx/N601/gongxue/jjsx/img/1-3/image020.png" alt="" align="middle">

<img src、"http://oss.ouchn.cn/ddzx/N601/gongxue/jjsx/img/1-3/image021.png" alt="" align="middle">