A lamp of negligible height is placed on the ground l₁ away from a wall. A man l₂m tall is walking at a speed of l₁.10 m/s from the lamp to the nearest point on the wall. When he is midway between the lamp and the wall, the rate of change in the length of this shadow on the wall is

a. -5l₂/2m/s
b. -2l₂/5m/s
c. l₂/2m/s
d. l₂/5m/s