I listened to Dr.Wilson's lecture about the protection of wild animals, but I was unable to grasp its key .
____ 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.
He gave us a nod and muttered a “yes” after _______ seemed like hours.
In any case, parents should make clear _______, if any thing, the children are expected to pay for with the money.