The resultant of two vectors `vec(P)` and `vec(Q)` is `vec(R )`. If the magnitude of `vec(Q)` is doubled, the new resultant vector becomes perpendicular to `vec(P)`. Then, the magnitude of `vec(R )` is equal to
A. `(P^(2)-Q^(2))/(2PQ)`
B. `(P+Q)/(P-Q)`
C. `Q`
D. `P/Q`