【单选题】
下列方程是常微分方程的是( )
①
<img src=https://huaweicloudobs.ahjxjy.cn/A3CCF902222C5F3D3BE56F98276D413E.png title=$9D(S{W]Q9`9OGAQ[E`N4Y4.png alt=$9D(S{W]Q9`9OGAQ[E`N4Y4.png/>
②
<img src=https://huaweicloudobs.ahjxjy.cn/4BDA6C1AAA4C3BEF722ED18102F63AF6.png title=9S6SM$0@C1]HYMGBJG4ZXAR.png alt=9S6SM$0@C1]HYMGBJG4ZXAR.png/>
③
<img src=https://huaweicloudobs.ahjxjy.cn/3BB54C16B2D33E5DBFA6127B1136CA4F.png title=MSTNEZ%J~{38WE}K`N[B93S.png alt=MSTNEZ%J~{38WE}K`N[B93S.png/>
④
<img src=https://huaweicloudobs.ahjxjy.cn/37260B65D2E3E54B0A9BDF6A6CB8A4DF.png title=OJWQGDXWBKQ7UC~GPT4O7NJ.png alt=OJWQGDXWBKQ7UC~GPT4O7NJ.png/>