If Two Collinear Vectors $\overrightarrow { \mathrm { A } }$

If two collinear vectors $\overrightarrow { \mathrm { A } }$ and $\vec { B }$ are added,the resultant has a magnitude equal to 4.0.If $\vec { B }$ is subtracted from $\overrightarrow { \mathrm { A } }$ ,the resultant has a magnitude equal to 8.0.What is the magnitude of $\overrightarrow { \mathrm { A } }$ ?

A) 2.0
B) 3.0
C) 4.0
D) 5.0
E) 6.0