Two cars $X$ and $Y$ traveling in opposite directions along the same highway at uniform velocities $110 \mathrm{Km} / \mathrm{h}$ and $90 \mathrm{Km} / \mathrm{h}$ respectively pass each other at a certain point. The velocity of $X$ relative to $Y$ at the time they pass each other is

(a) $200 \mathrm{Km} / \mathrm{h}$

(b) $100 \mathrm{Km} / \mathrm{h}$

(c) $40 \mathrm{Km} / \mathrm{h}$

(d) $20 \mathrm{Km} / \mathrm{h}$.