Which of the following differential equations is/are nonlinear?
- $t x(t)+\frac{d x(t)}{d t}=t^{2} e^{t}, \quad x(0)=0$
- $\frac{1}{2} e^{t}+x(t) \frac{d x(t)}{d t}=0, \quad x(0)=0$
- $x(t) \cos t-\frac{d x(t)}{d t} \sin t=1, \quad x(0)=0$
- $x(t)+e^{\left(\frac{d x(t)}{d t}\right)}=1, \quad x(0)=0$