Vacation is coming. He is thinking about .
A：where he goes
B：where to go
C：where is he going to
D：where going
②检查装置的气密性的方法是：____；
③检验氧气的方法是：____；
④空气的成分以____ 和____为主，其中按体积分数计，____约占空气体积的78%，____约占空气体积的21%．
⑤玻璃仪器洗净的标准是____．
—I think it was not good．It was a(n) performance．
