In Hindley-Milner type inference, what does the occurs check in unification prevent, and what happens if it is omitted?

A It prevents two distinct type variables from ever being unified with each other under any circumstances B It prevents the algorithm from generalizing a type into a polymorphic let-binding C It prevents function types from being unified with non-function types during constraint solving D It prevents a type variable from being unified with a type expression that contains that same variable, which would otherwise build an infinite (cyclic) type and cause the algorithm to loop forever or produce an inconsistent substitution