21.(2021•全国Ⅲ卷模拟)已知ΔABC\Delta ABCΔABC的内角AAA,BBB,CCC所对的边分别为aaa,bbb,ccc.ΔABC\Delta ABCΔABC内一点MMM满足:a⋅MA→+b⋅MB→+c⋅MC→=0a\cdot \overrightarrow{MA}+b\cdot \overrightarrow{MB}+c\cdot \overrightarrow{MC}=0a⋅MA+b⋅MB+c⋅MC=0,则MMM一定为ΔABC\Delta ABCΔABC的((( )))