【单选题】
下列方程是常微分方程的是( )
①
<img src=https://huaweicloudobs.ahjxjy.cn/12103E9126D544E14AC06D83ABC68690.png title=8OTBW[X7@Q2WKDTH~DCD}9O.png alt=8OTBW[X7@Q2WKDTH~DCD}9O.png/>
②
<img src=https://huaweicloudobs.ahjxjy.cn/96B6A202D02AB5BCE0940018BC088804.png title=36NGOZUQ_5A%T09%QOS5()O.png alt=36NGOZUQ_5A%T09%QOS5()O.png/>
③
<img src=https://huaweicloudobs.ahjxjy.cn/9EF7820B824292764655EE85700A8457.png title=A@_2{TI1KUV(H$~Z4TLTZ_D.png alt=A@_2{TI1KUV(H$~Z4TLTZ_D.png/>
④
<img src=https://huaweicloudobs.ahjxjy.cn/2A61ADB4A3D244026070805B5774D53B.png title=GJW{S0BSV3D0}8)QK0BIQUJ.png alt=GJW{S0BSV3D0}8)QK0BIQUJ.png/>