C-12-Q037mediumsingle_mcqIn △ABC\triangle ABC△ABC, AB⃗+BC⃗+CA⃗=\vec{AB}+\vec{BC}+\vec{CA}=AB+BC+CA=aAC⃗\vec{AC}ACb0⃗\vec{0}0c2AB⃗2\vec{AB}2ABdCA⃗\vec{CA}CA