Knock off vs. knock up?

[Lolololo]

So please if knock off and knock up have the same meaning ( to produce something quickly ) why we used knock up.. the both are right, aren't?

[Thredder]

To add to the confusion, I think you could also say "I have lots of material, I can knock something together."

"Knock together" would mean that you put the materials together to create something (quickly and often carelessly).
"knock up" would mean that based on the materials (though not necessarily just using them) you could create something new.
"knock off" would mean that you created something that was a copy of the material.

for me, 'knock off is the least likely of the three answers. I think the correct answer is probably meant to be 'knock up', though 'knock together' would not be wrong (though the meaning would be slightly different).

[Our Tort System]

[Cornel]

So .... Is "knock off" a wrong answer after all?
The meaning of "knock off" is quite close to the meaning of "knock up" with some differences of nuance, issued form the context. "To knock off" can mean "to produce, to accomplish, to complete, to produce somtehing quickly". With this meaning, "knock off" could be as correct as "knock up" in the context of the given question. Am I wrong? Maybe for a non-native english speaker it's harder to understand these differences or maybe both asnwers could be correct...

Source for translations:
http://idioms.thefreedictionary.com/knock+off
http://dictionary.cambridge.org/dictionary/british/knock-sth-off_2

Dear teachers, I'm waiting for more explanations if you're so kind. Thank you before!