如图，在直角三角形ABC中，∠ACB=90°，D是AB上一点，且∠ACD=∠B．

求证：CD⊥AB．

在鸽子卵的结构里，卵黄表面中央有个小白点，内有细胞核，这个结构称为（   ）

B

    Scientists say life will be very different in 2050.

    First, there will be no TV channels (频道) in 2050. People will choose a program (节目) from a computer and watch it on TV. In 2050, we will get information from the computers only.

    Second, water will be a big problem in 2050. In many places, people won't have water to drink.

    Third, cars will use new and clean fuels (燃料) and they will go very fast. There will be a computer in every car. And the computers will control the cars and there won't be any accidents (事故).