Amit started cycling along the boundaries of a square field from cover point $A$. After half an hour, he reached the comer point $C$, diagonally opposite to $A$. If his speed was $8 \mathrm{~km} / \mathrm{hr}$, what is the area of the field in square $\mathrm{km}$
(a) 64
(b) 8
(c) 4
(d) Cannot be determined