4. It used to ______ that humans and animals were different and had little in common.

5. The day breaks, _______ the birds are singing.

6. _________, I attended the meeting instead of him last week.

7. When you read the book, you’d better make a mark ______ you have questions.

10. By the middle of the 21st century , the vast majority of the world’s population ________ in cities rather than in the country.

11. It is because people are likely to stay indoors ______ the virus can spread easily ______ people often catch colds.

12. He gave us a nod and muttered a “yes” after _______ seemed like hours.

13. In any case, parents should make clear _______, if any thing, the children are expected to pay for with the money.

14. Many new games are designed on the computers all the time; people of all ages ______ these games.

15. --- Who are the teenagers gathering at the school gate?

--- Well, if you _____ know, they are all my visiting students.