C

    Kevin and his family had a holiday to his hometown—a very beautiful island. It was a great trip.

    They all wanted this trip. Kevin's parents wanted to go out into nature after a long time of work. Kevin needed to take a good rest before the exam. Kevin's brother Jim is in the first grade, so he doesn't have a voice in family things. He was just happy that they were going outside.

    The sea was blue. They went boating in the sea. It was so cool. There was no Internet, so they spent the whole time together. They stayed in a small wooden house by the sea. Mom and Kevin cooked delicious sea food. Jim ran after chickens and ducks, and gave them food. When he became tired, he sat next to his dad and helped him fish. He even helped to hook two fish!

    It is always good to spend time with families. They liked this trip!     

缩句。

①北方的天气太冷了。

②学校里的学生认真地把教室的桌子擦干净。

③他狠狠地瞪了她一眼。

为安全起见，初学游泳者常使用一块泡沫浮板，用双臂把浮板压入水中，借助浮板所受的浮力来辅助游泳．已知泡沫浮板的密度是0.1×10³kg/m³，体积为3000cm³．（ρ_水＝1.0×10³kg/m³，g=10N/kg）求：
（1）当浮板自由漂浮在水面上时，浮板受到的浮力的大小；
（2）浮板被压没入水中静止时，浮板受到双臂的压力的大小。