Suppose the charge of a proton and an electron differ slightly. One of them is $-e$, the other is $(e+\Delta e)$. If the net of electrostatic force and gravitational force between two hydrogen atoms placed at a distance $d$ (much greater than atomic size) apart is zero, then $\Delta e$ is of the order of [Given mass of hydrogen $m_h=1.67\times 10^{-27}\text{kg}$ ]:

1. A
   $10^{-20}\text{C}$
2. B
   $10^{-23}\text{C}$
3. C
   $10^{-37}\text{C}$
4. D
   $10^{-47}\text{C}$