The speed of a boat in still water is $8 \mathrm{~km} / \mathrm{hr}$. If its speed downstream be $15 \mathrm{~km} / \mathrm{hr}$, then speed of the stream is:
(a) $7.5 \mathrm{~km} / \mathrm{hr}$
(b) $7 \mathrm{~km} / \mathrm{hr}$
(c) $9 \mathrm{~km} / \mathrm{hr}$
(d) None of these