Our city has developed into a big city,which is it used to be.
When you read the book, you’d better make a mark ______ you have questions.
I didn’t see your sister at the cinema. If she ______, she would have met my brother.
____ all behavior is learned behavior is a basic assumption of social scientists.
By the middle of the 21st century , the vast majority of the world’s population ________ in cities rather than in the country.
It is because people are likely to stay indoors ______ the virus can spread easily ______ people often catch colds.