Các tỉ số nào sau đây lập thành một tỉ lệ thức?

$a = c$

$a.c = b.d$

$a.d = b.c$

$b = d$

$\dfrac{5}{{35}} = \dfrac{9}{{63}}$

$\dfrac{{63}}{9} = \dfrac{{35}}{5}$

$\dfrac{{35}}{9} = \dfrac{{63}}{5}$

$\dfrac{{63}}{{35}} = \dfrac{9}{5}$

$\dfrac{5}{{15}} = \dfrac{{ - 9}}{{ - 27}};\,\dfrac{{15}}{5} = \dfrac{{ - 27}}{{ - 9}};\,\dfrac{5}{{ - 9}} = \dfrac{{15}}{{ - 27}};\,\dfrac{{ - 9}}{5} = \dfrac{{ - 27}}{{15}}$

$\dfrac{5}{{15}} = \dfrac{{ - 9}}{{ - 27}};\,\dfrac{{15}}{5} = \dfrac{{ - 27}}{{ - 9}};\,\dfrac{5}{{ - 9}} = \dfrac{{15}}{{ - 27}};\,\dfrac{{ - 9}}{5} = \dfrac{{ - 15}}{{27}}$

$\dfrac{5}{{15}} = \dfrac{{ - 9}}{{ - 27}};\,\dfrac{{15}}{5} = \dfrac{{ - 27}}{9}$

$\dfrac{{15}}{5} = \dfrac{9}{{27}};\,\dfrac{{15}}{5} = \dfrac{{ - 27}}{{ - 9}};\,\dfrac{5}{{ - 9}} = \dfrac{{15}}{{ - 27}};\,\dfrac{{ - 9}}{5} = \dfrac{{ - 15}}{{27}}$

$\dfrac{2}{a} = \dfrac{5}{b}$

$\dfrac{b}{5} = \dfrac{2}{a}$

$\dfrac{2}{5} = \dfrac{a}{b}$

$\dfrac{2}{b} = \dfrac{5}{a}$

$x = \dfrac{1}{5}$    

$x =  - \dfrac{5}{4}$

$x = \dfrac{5}{4}$

$x = \dfrac{4}{5}$

$1$

$2$

$0$

$3$