Which of the following propositions are so related that they cannot both be true, although they may both be false?

1.All frogs are amphibians

2.Some frogs are amphibians

3.Some frogs are not amphibians

4.No frogs are amphibians

A1 & 3 only

B3 & 4 only

C2 & 3 only

D1 & 4 only ✓ Correct

Correct answer: (D) 1 & 4 only

Explanation

Statement 1 and Statement 4 cannot both be true though they may both be false, making them contraries.

Statement 1 is the universal affirmative A: All frogs are amphibians.

Statement 4 is the universal negative E: No frogs are amphibians.

Two universals of opposite quality on the same terms are contraries.

Contraries rule out joint truth but allow joint falsity, exactly the relation asked.

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