—He's my classmate.
A:Who
B:Where
C:Whose
D:What
① AB→−AC→=BC→ ;
② AB→+BC→+CA→=0→ ;
③若 (AB→+AC→)·(AB→-AC→)=0 ,则△ABC为等腰三角形;
④若 AC→·AB→>0 ,则△ABC为锐角三角形.
上述命题正确的是( )