一、根据句意和汉语提示，写出所缺单词的正确形式（每小题2分）
1. Building such a skyscraper is not an easy job; we must have enough _____________(专业的) engineers.
2. It was by accident that he broke the valuable vase. He didn’t do that _______________ (故意地).
3. He _____________ (获得)a knowledge of computers by careful study.
