I agree with the last method that people participate in many different activities for a few reasons.
One reason is my memory that I think do many activities is better than focus in one activity for people’s intelligence development. Because I do many activities (listening, reading, speaking, writing and typing activities) before I apply for TOEFL test. So I think these activities develop my skill and lead to develop my intellect.
Another reason is my friend. When I practice speaking and writing activities with my friend. I was trained to develop my intelligence quotient and adversity quotient. Furthermore we will develop our relationship.
Therefore, I agree that people participate in many different activities to develop their intellect.