2014-II-25

Ans: D
I is true. According to the given figure, the slope of the straight line $ax+by=1$ is positive. Hence, we have

$\begin{array}{rcl}
\dfrac{-a}{b} & > & 0 \\
-a & > & 0 \\
a & < & 0 \end{array}$

II is true. According to the given figure, the slope of the straight line $cx+dy=1$ is negative. Hence, we have

$\begin{array}{rcl}
\dfrac{-c}{d} & < & 0 \\ -c & < & 0 \\ c & > & 0
\end{array}$

III is true. According to the given figure, the $y$ intercepts of the two straight lines $ax+by=1$ and $cx+dy=1$ are the same. Hence, we have

$\begin{array}{rcl}
\dfrac{1}{d} & = & \dfrac{1}{b} \\
b & = & d
\end{array}$