Study the flow chart given below.
What can be placed at P, Q, R and S?
| P | Q | R | S | |
| A | Window pane | Ceramic tile | Iron rod | Spectacle lens |
| B | Iron rod | Rubber band | Ceramic tile | Spectacle lens |
| C | Iron rod | Rubber band | Window pane | Ceramic tile |
| D | Ceramic tile | Window pane | Rubber band | Iron rod |
How can a window pane stretched ?