一根旗杆在阳光照射下，在地面上投下一个旗杆的影子．自早晨到黄昏这段时间内，旗杆影子长度变化是  （  ）

2011年5月22日，世界内河上最豪华的邮轮——“长江黄金l号”在重庆朝天门下水首航。如图所示，一架重力为9×l0⁴N的直升飞机在20s内竖直下降10m，最后停在邮轮的停机坪上。 求： 

阅读短文，然后根据其内容从A、B、C、D中选出最佳选项。

Mr Li: Sam, did you enjoy your first Spring Festival in China?

Sam: Yes, I did. During this festival, many family members get together.

Mr Li: Yes. I hear that at Thanksgiving Americans also get together for a special meal. It sounds similar.

Sam: Yes. It's similar. During these holidays, both the Chinese and Americans come home and have a big dinner. It's great fun.

Mr Li: What do you think are the main differences between the Spring Festival and Thanksgiving?

Sam: Well, we take part in different activities and eat different kinds of food. In China, people usually set off fireworks and eat dumplings. In the US, we usually watch a Thanksgiving parade and an American football game on TV. We also eat turkey.

Mr Li: That's interesting.

①增大压强　②增加炭的量 ③通入CO₂　④恒压下充入N₂　⑤恒容下充入N₂　⑥升温