Final answer:
Mrs. Drysdale claimed they couldn't stay when the doorbell rang because she wanted to be polite.
Step-by-step explanation:
In the play, Mrs. Drysdale claimed they couldn't stay when the doorbell rang because she wanted to be polite. This can be seen when she said, 'We just wanted to be polite.' It is likely that she did not want to seem rude by staying when someone was at the door, so she used the excuse of having another appointment or an urgent matter to justify their departure.