A lantern is placed on the ground 100 feet away from a wall. A man six feet tall is walking at a speed of 10 feet/second from the lantern to the nearest point on the wall. When he is midway between the lantern and the wall, the rate of change in the length of his shadow is\\
(a) $3.6 \mathrm { ft } . / \mathrm { sec }$.\\
(b) $2.4 \mathrm { ft } . / \mathrm { sec }$.\\
(c) $3 \mathrm { ft } . / \mathrm { sec }$.\\
(d) $12 \mathrm { ft } . / \mathrm { sec }$.