A:甲
B:乙
C:丙
D:无法判断
Come and visit England—museums and parks
British Museum
Subway: Tottenham Court Rd
You can get the information about Rosetta Stone inscriptions (碑文)here.
You can also take a walk in Great Court. It is the biggest indoor square in Europe.
National Library
Subway: Charing Cross
You can see more than 2000 famous paintings from the famous Europeans here.
It is one of the largest art galleries in the world.
Natural History Museum
Subway: South Kensington
You can experience the fearful time. It brings you to the terrible time of the dinosaurs.
You can see other wonders of nature here with your own eyes.
Greenwich Park
Light rails
You can see the beautiful roses here.
It is London's oldest royal(皇家的) park with the longest history. The Royal Weather Station is here, too.
如图所示为两组平行板金属板,一组竖直放置,一组水平放置,今有一质量为m的电子静止在竖直放置的平行金属板的A点,经电压U0加速后通过B点进入两板间距为d、电压为U的水平放置的平行金属板间,若电子从两块水平平行板的正中间射入,且最后电子刚好能从右侧的两块平行金属板穿出,A、B分别为两块竖直板的中点,求: