1. It was raining heavily when he ________ home.
2. Mary wonders if her uncle ________ his service as a guide when she visits Beijing.
3. —Amy, I called you yesterday evening, but nobody answered the phone.
    — Oh, I ________ a walk with my mother at that time.
4. —________ you ________ a letter to your mother recently?
    —Yes, I wrote one last Sunday.
5. We are told that sound ________ through air at a speed of about 340 metres per second.