顶棚抹灰发生空鼓或裂缝的主要原因是：基层清理不干净，一次抹灰过厚，配制砂浆或原材料质量不好或使用不当。( )
参考答案：对
─Where can I find Jack?
─He _____ the post office. [ ]
A. has been to
B. had been to
C. has gone to
D. had gone to
钳工常用工具有手锤、钳子、锯、水平仪等。
