A. $\underset{AB}{\to }-\underset{AC}{\to }=\underset{BC}{\to }$

B. $\underset{AB}{\to }+ \underset{AC}{\to }=\underset{BC}{\to }$

C. $\underset{AB}{\to }+\underset{AC}{\to }=2\underset{AD}{\to }$

D. $\underset{AD}{\to }+\underset{BE}{\to }-\underset{CF}{\to } =0$