A dynamic $\text{RAM}$ cell which hold $5 \mathrm{~V}$ has to be refreshed every $20 \mathrm{~m}$ secs, so that the stored voltage does not fall by more than $0.5 \mathrm{~V}$. If the cell has a constant discharge current of $0.1 \; \mathrm{pA}$, the storage capacitance of the cell is
- $4 \times 10^{-6} \mathrm{~F}$
- $4 \times 10^{-9} \mathrm{~F}$
- $4 \times 10^{-12} \mathrm{~F}$
- $4 \times 10^{-15} \mathrm{~F}$