如图所示，AOB是光滑水平轨道，BC是半径为R的光滑的 $\frac{1}{4}$ 固定圆弧轨道，两轨道恰好相切于B点．质量为M的小木块静止在O点，一个质量为m的子弹以某一初速度水平向右射入小木块内，并留在其中和小木块一起运动，且恰能到达圆弧轨道的最高点C（木块和子弹均可以看成质点）．求：

根据短文内容及首字母提示完成短文。

    My name is John. I am 13 years old. My b ____is 1st October and I was b____ in Nanjing. I live with my f____ in Shanghai. I have a t____ brother. I am s____ and I w____ glasses. I am good at English. I like playing football. I play football in the football f____. I also like c____ games. In the evening, I do my homework carefully. My father r____ newspapers. My mother c____dinner for us. My brother reads Chinese books every day.

振动越快，发出的声音就越高。（    ）

空气中含量较多且化学性质不活泼的气体是（    ）