A man standing on a road has to hold his umbrella at $30^{\circ}$ with the vertical to keep the rain away. He throws the umbrella and starts running at 10 km/h. He finds that raindrops are hitting his head vertically. The actual speed of raindrops is :

Question

\[\begin {array} {1 1} (a)\;20 km/h & \quad (b)\;10 \sqrt 3 km/h \\ (c)\;20 \sqrt 3 km/h & \quad (d)\;10\;km/h \end {array}\]