[单选题]
A stream flows east with a speed $v_s$. A boat points due south and moves with a speed $v_o$ relative to the water.
The ratio $\frac{v_o}{v_s}=\frac{1}{2}$
Which of the following is correct about the magnitude in $ms^{-1}$ and the angle $\theta$ between the velocity of the boat and the side of the stream?
A. Magnitude is $\sqrt{3}v_o\:\theta\:is \sin^{-1}(\frac{1}{2})$
B. Magnitude is $\sqrt{3}v_o\:\theta\:is \cos^{-1}(\frac{1}{2})$
C. Magnitude is $\sqrt{5}v_o\:\theta\:is \tan^{-1}(2)$
D. Magnitude is $\sqrt{5}v_o\:\theta\:is \tan^{-1}(\frac{1}{2})$
