Speed of a man is $10 \mathrm{~km} / \mathrm{hr}$ in still water. If the rate of current is $3 \mathrm{~km} / \mathrm{hr}$, then the effective speed of the man upstream is:
(a) $7 \mathrm{~km} / \mathrm{hr}$
(b) $8.5 \mathrm{~km} / \mathrm{hr}$
(c) $9 \mathrm{~km} / \mathrm{hr}$
(d) None of these