轻质杠杆每小格的长度相等，O为支点．在杠杆左侧挂一物体甲，若在杠杆右侧挂一物体乙，并在物体乙的下方拴接一个弹簧，如图所示，当杠杆在水平位置时，整个装置处于平衡状态．已知物体乙的重力为600N，弹簧对物体乙的作用力为300N．下列说法正确的是（　　）

填写单词 根据句子意思, 在空格中填写适当的单词, 使句子完整、正确。

Here are four seasons in a year — spring, ____ , autumn and winter.

Frank was ____ in 2000. He is fifteen this year.

It's 12:00 at noon. It's time for students to have____ in the dining hall.

To keep____ , you'd better exercise every day and eat more fruit and vegetables.

In China, many children are taken____ of by their grandparents in the village.

— I've ____ been to a water park before.

— Me neither.

What does John want to be when he____ up?

A boy ____  himself in P. E. class. The teacher and two students took him to the  hospital at once.

下图所示的实验中，发生了化学变化的是（）