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1. Skateboarding became very popular in the 1970s. (In the 1970s. skateboarding suddenly became very popular)

2. They are 'freestyle' skateboarding, 'ramp' skateboarding, and 'street-style' skateboarding. (This is called 'freestyle' skateboarding...ramp)

3. It is a combination of freestyle and ramp. (This was 'street-style' skateboarding - a combination of freestyle and ramp)

4. Because this allowed them to skateboard safely. (For this, the skateboarders needed protective clothing such as...safely)

5. Yes, I do. Because many people want to consider it their hobby now.