题干
Mike is a student at a famous university, _______ for a doctor degree.
A：studied
B：studying
C：to have studied
D：to be studying
答案(点此获取答案解析）
B
A：studied
B：studying
C：to have studied
D：to be studying
为坐标平面内三点，O为坐标原点，若
方向
满足的关系式为 （ ）