If $y$ is a solution of $y(x)-\int_{0}^{x}(x-t) y(t) d t=1$
1. $y$ is bounded but not periodic in $\mathbb{R}$
2. $y$ is periodic in $\mathbb{R}$
3. $\int_{\mathbb{R}} y(x) d x<\infty$
4. $\int_{\mathbb{R}} \frac{d x}{y(x)}<\infty$