The Gini coefficient $(index)is$ a measurement $of$ income inequality. Its value ranges between $0$ and $1,$ where $0$ indicating perfect equality and $1as$ perfect inequality. $To$ calculate the Gini coefficient, $we$ need the Lorenz Curve which graphically shows the distribution $of$ income. $Itis$ usually represented $as$ the portion $of$ income $(Y-$axis$)to$ the percentage $of$ population $(X-$axis$).$ Here $we$ name the Lorenz Curve $as$ the $C-$Line, and the Gini coefficient $as(Gamma).$ With the information given, try $to$ finish the following questions.

$(1)To$ calculate Gamma, $we$ also need $a45-$degree diagonal line within the diagram $of$ income distribution. Suppose $we$ label this diagonal line $asD-$Line. What the $D-$Line represent for?