欧洲航海者的日记显示，在15世纪以前，毛里求斯岛上的渡渡鸟随处可见。后来欧洲殖民者到达了毛里求斯岛，他们疯狂地捕猎肉味细嫩鲜美的渡渡鸟，加之殖民者带来的猪、狗、鼠等，渡渡鸟灭绝了。渡渡鸟灭绝之后，毛里求斯岛上的卡尔瓦利亚树也面临灭绝的危险。这种树的果皮坚厚，种子很难萌发。渡渡鸟最喜欢吃卡尔瓦利亚树的果实，果实经过渡渡鸟的消化道后，果皮变薄了，果实中的种子得以正常萌发。

阅读下列短文，从每小题A、B、C、D四个选项中选出一个最佳答案。

Every year at our school, we have a special day called International Day (国际日). It is for students to learn about different cultures (文化) from different countries. On this day, each class must tell other classes about a different country. Of course you will see many dances and different foods.

If you visit the classroom of Japan, you can play the games Japanese children play. You also have some special Japanese foods. In the classroom of Mexico (墨西哥), there are tomalleys (龙虾肝) and chips to eat.

    In the afternoon, all the students, teachers and parents watch the dances. There are lots of dances.

    These dances are beautiful. People enjoy the shows.

    I enjoy International Day very much.

圆锥的底面半径是10cm，高是2cm．

①这个圆锥的底面周长是____；

②这个圆锥的底面积是____  $c{m}^2$ ；

③这个圆锥的体积是____  $c{m}^3$  

如图所示的电路中，P为滑动变阻器的滑片，保持理想变压器的输入电压U₁不变，闭合开关S，下列说法正确的是（   ）