下列方程中为常微分方程的是( )<br>·<img src="/lqwvjeImg/5f/5f319a83e453a1361b3ec1db83b579a3.png"><br>·<img src="/lqwvjeImg/23/232ea00298b978674205633b6597c763.png"><br>·<img src="/lqwvjeImg/ca/ca1f143cb61e39f86302f9bb25618af6.png"><br>·<img src="/lqwvjeImg/39/39bf40f97ed0f657ebf1b6c6e193db47.png">(c为常数)