The key answer of exercise 4

a)

1. at
2. to
3. at
4. in
5. on

d)

1. the weekend → at the weekend
2. the morning → in the morning
3. seven o’clock → at seven o’clock
4. Sunday → on Sunday
5. night → at night
6. the evening → in the evening
7. midnight → at midnight
8. the summer → in the summer
9. Tuesday morning → on Tuesday morning
10. Christmas → at Christmas