Classical music is serious music．People usually enjoy it at a c____or in a theatre．Beethoven and Mozart are great c____musicians．

    Pop music often comes and goes quickly．It is usually about love and everyday life．Country music，____(摇滚)music and jazz are all pop music．They are very popular with young people．

    Folk music is ____(简单的)but special．Guo Lanying，Song Zuying and Tengger

 are____(著名的)for their folk songs．

电子以初速度v₀沿垂直场强方向射入两平行金属板中间的匀强电场中，现增大两板间的电压，但仍使电子能够穿过平行板间，则电子穿越平行板所需要的时间（   ）

已知函数$f\left(x\right)=\frac{x^2+x+1}{x^2+1}$，若$f\left(a\right)=\frac{2}{3}$，则f（﹣a）=（　　）