Read the text, complete the sentence and choose the correct answer.

If you want to get in good physical shape, extreme sports may be your wise choice. Skateboarding is a prime example because an average person burns as much as 500 calories per hour, when he rides a skateboard. Of course, you will get better results, if you work on the complicated surfaces doing skateboarding tricks instead of merely riding on the flat surfaces. Moreover, they are more exciting than standard exercises.

Riding a skateboard on the flat surface is less exciting than ...

• riding a skateboard on the uneven surface.
• riding a skateboard on the viewing surface.
• riding a skateboard on the tarmac surface.
• riding a skateboard on the wet surface.