良性骨肿瘤的X线表现为（）。
A.边缘清楚
B.出现葱皮现象
C.骨膜反应明显
D.空腔形成
E.多处呈虫蛀状破坏
参考答案：A
We are taught that a business letter should be written in a formal style ________ in a personal style. [ ]
A. rather than
B. other than
C. better than
D. more than
根据速度轴系和机体轴系的相互关系，可以确定直升机的姿态角有哪些？
