A thin plastic rod is bent into a circular ring of radius $R$. It is uniformly charged with charge density $\lambda$. The magnitude of the electric field at its centre is : (A) $\frac{\lambda}{2\varepsilon_0 R}$ (B) Zero (C) $\frac{\lambda}{4\pi\varepsilon_0 R}$ (D) $\frac{\lambda}{4\varepsilon_0 R}$