[LHM]

Hi, I'm still pretty new to this place, I just found it like yesterday, so sorry if I messed things up and this is actually supposed to go in the organic forum. I wasn't sure because this is from a high school based source. So go ahead and move it if it doesn't go here.

Which of these compounds have cis- and trans- isomers?
 A) dichlorobenzene
 B) 1-chloropropene
 C) 1,2-dichloropropane
 D) dichloroethyne

I already know that it's not A, and I do know how to draw out these structures. Out of the rest of them though, I don't really see how come C and D don't have cis- and trans- isomers, while B (which is the answer) does. Maybe there's a flaw in my understanding of what makes a cis- and trans- isomers? But if someone could explain it, I'd really appreciate it

[Schrödinger]

Perhaps this will help :
http://en.wikipedia.org/wiki/Cis%E2%80%93trans_isomerism