L and M are two points with position vectors `2vec(a)-vec(b)andvec(a)+2vec(b)`, respectively. The position vector of the pont N which divides the line segment LM in the ratio 2:1 externally is
A. `3bar(b)`
B. `4bar(b)`
C. `5bar(b)`
D. `3bar(a)+4bar(b)`