A dynamic RAM cell which holds 5 V has to be refreshed every 20 ms so that the stored voltage does not fall by more than 0.5 V. If the cell has a constant discharge current of 0.1 pA, the storage capacitance of cell is

• A. 4 x 10‾⁶ F
• B. 4 x 10‾⁹ F
• C. 4 x 10‾¹² F
• D. 4 x 10‾¹⁵ F