如图所示是倾角θ=45°的斜坡，在斜坡底端P点正上方Q点以速度v₀水平向左抛出一个小球A，小球恰好能以垂直斜坡的速度落在斜坡上，运动时间为t₁．小球B从同一点Q自由下落，下落至P点的时间为t₂．不计空气阻力．求t₁与t₂的比值．

如图表示蛋白质结构的一部分，图中A、B、C、D标出分子中不同的化学键，当蛋白质发生水解反应时，断裂的化学键是（　　）

Betty：Hi, Mum!

Mum：Hello, Betty! Where are you now?

Betty: I'm____ (stand) on the Great Wall of China and____ (talk) to you. Mum：Really? That's great.

Betty：We're on a school trip.

Mum: What about the____(other)? Are they with you?

Betty: Well, right now Tony is ____(eat) a delicious ice cream. Wang Hui is ____take) lots of photos. Daming ____(have) lunch in the sun. And just now Lingling _____(buy) a few presents and postcards. Because they ____(be) on sale in the shop.

Mum: I'd like a postcard too, and please take some photos of the Great Wall and send____(they) to me by email.

Betty: OK. We're enjoying the school trip a lot. Anyway, it's time____(go) back to school now. Bye, Mum!

Mum：Bye, Betty!