A dog is tied to a tree by a long nylon cord. If the dog runs from the due North side of the tree to the due South side

The dog runs from the North side to the South side of the tree (semi-circle) with the cord to its full length at all times.
The dog runs approximately 30 feet.
Target question: what was the approximate length of the nylon cord, in feet?
$$Circumference\ of\ a\ fall\ circle=2\pi r$$
$$Circumference\ of\ a\ semi\ circle=\frac{2\pi r}{2}=\pi r$$
Since the dog covered 30 feet
$$\pi r=30;\ \ Take\ \pi\ to\ be\ 3.142$$
So, 3.142r = 30
$$r=\frac{30}{3.142}=9.55\approx10feet$$
Therefore, option D is the correct answer