Match List I with List II.
List I | List II | ||
| A | Taenia | I | Nephridia |
| B | Paramoecium | II | Contractile vacuole |
| C | Periplaneta | III | Flame cells |
| D | Pheretima | IV | Urecose gland |
Choose the correct answer from the options given below:
Two charges $ -q $ each are fixed, separated by distance $ 2d $. A third charge $ q $ of mass $ m $ placed at the mid-point is displaced slightly by $ x' (x \ll d) $ perpendicular to the line joining the two fixed charges as shown in the figure. The time period of oscillation of $ q $ will be: