[单选题]
A box is pulled up a slope inclined at $30^{\circ}$ to the horizontal by a force of $F$.The angle between the direction of $F$ The angle between the direction of $30^{\circ}$.
The box is pulled a distance of d up along the slope. What is the work done by F on the box?
A. $Fd\,\cos (30^{\circ})$
B. $Fd\,\cos(60^{\circ})$
C. $Fd\,\sin(30^{\circ})$
D. $Fd\,\cos^2(30^{\circ})$
