If \(\rm \vec a, \vec b, \vec c\) are unit vectors such that \(\rm \vec a\) is perpendicular to the plane of \(\rm \vec b, \vec c\); and the angle between \(\rm \vec b\) and \(\rm \vec c\) is \(\frac{\pi}{3}\). Then, what is \(\rm |\vec a+\vec b+\vec c|\)?
1
1
2
2
3
3
4
4