For a van der Waals gas at low pressure the Joule-Thomson coefficient is $\mu =\frac{1}{C_p}\left(\frac{2a}{RT}-b\right)$. Which statements are correct?

1. A$\mu$ is positive when $T$ is less than $2a/Rb$
2. B$\mu$ is negative when $T$ is greater than $2a/Rb$
3. CFor a gas with $a=0$, $\mu$ is negative at all temperatures
4. DFor a gas with $b=0$, $\mu$ is negative at all temperatures