Select the Fallacy or Bad Argument Technique Which Occurs in the Following

Multiple Choice

Select the fallacy or bad argument technique which occurs in the following argument: We should preserve marriage as a relationship between a man and a woman. If we undercut traditional ideals of marriage by allowing gay people to marry, before you know it we'll be allowing polygamous relationships, and then incestuous ones, and people will end up being able to marry their own brothers and sisters! No gay marriage!

A) Post hoc ergo propter hoc
B) Smokescreen
C) Fallacy of denying the antecedent
D) Slippery slope