If the proposition Some frogs are not mammals is given as false, then which of the following propositions can be immediately inferred to be false?

No frogs are mammals must be false when the O proposition is false.

Some frogs are not mammals is the O statement declared false.

The contradictory of O is the universal affirmative A, which is therefore true.

A true A, All frogs are mammals, has as its contrary the E form, which must be false.

So No frogs are mammals is the statement inferred false.