Answer: They wanted to prove that segregated buses were illegal.
Explanation: They want to bring it to court but they need the arrest of black rider so they have something to say that is illegal they shouldn't get arrested for something like that and that its illegal.
it is a simile.
a figure of speech involving the comparison of one thing with another thing, using like or as
The only powder that should be used in a muzzleloader is black powder.
