气密性试验主要是检验容器或管道系统的各联接部位是否有（）现象。
A、泄漏
B、裂痕
C、损伤
D、超压
参考答案：A
Though _____ many times, the song still sounds as nice as before [ ]
A. it was heard
B. hearing
C. heard
D. to be heard
生活垃圾分类工作的原则？
