Do you like animals? There are a lot of animals in the 1. They are pandas, koalas, tigers, lions, dolphins, penguins, elephants and so on. I think dolphins are very 2. They are kind of3. When I'm free, I 4 watching them. They can 5 fast(快)and jump very high(高). They can 6 with a ball. They can “7” up and “walk” on the water. They are very8 to people. If you fall into(掉进)the water and9 swim, they may come up and10 you. Do you like them?

（2012·深圳） 如图所示，木箱重600N，小明用F=200N的力沿水平方向推木箱，木箱做匀速直线运动。当推力增大到240N时，此时的摩擦力为（    ）