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In the figure above, line $ l $ passes through the point $ \left( 0,0 \right) $ and the two points indicated in the graph. What is the value of the product $ ab $, where $ a $ and $ b $ are constants?

$ -\cfrac { 1 }{ 2 } $

$ \cfrac { 1 }{ 2 } $

$ -2 $

$ 2 $