The density of a gas at STP is 2g/l. Its molecular weight is

Using the equation, $\mathrm{\rho }=\frac{\mathrm{PM}}{\mathrm{RT}}$

​⇒$\mathrm{M}=\frac{\mathrm{\rho RT}}{\mathrm{P}}$

​⇒$\mathrm{M}=\frac{2\times 0.0821\times 273.15}{1}$

​⇒M=44.8 g

Therefore, the correct option is C.