The graph of the line $a x + b y = c$ is drawn, where $a , b$ and $c$ are real non-zero constants.
Which one of the following is a necessary but not sufficient condition for the line to have a positive gradient and a positive $y$-intercept?
A $\frac { c } { b } > 0$ and $\frac { a } { b } < 0$
B $\frac { c } { b } < 0$ and $\frac { a } { b } > 0$
C $a > b > c$
D $a < b < c$
E $\quad a$ and $c$ have opposite signs
F $\quad a$ and $c$ have the same sign