Read the following paragraph and answer the question which follows. Did you know that lions are friendly? Lions are very sociable, at least with each other. They live in groups called "prides." Once in a pride, the females stay together for life. As a result, the females are able to pass on important knowledge to the cubs, including the pride's territory and the best places to hunt and find water. The females do all the hunting for food. However, the males protect the pride. A lion pride is like a large family.

Which sentence is the introductory sentence?

Did you know that lions are friendly?
Lions are very sociable, at least with each other.
They live in groups called "prides."
Once in a pride, the females stay together for life.