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

1.All animals are birds

2.Some animals are birds

3.No animals are birds

4.Some animals are not birds

AI and III only

BI and II only

CIII and IV only

DII and IV only ✓ Correct

Correct answer: (D) II and IV only

Explanation

Statement 2 and Statement 4 are sub-contraries and form the required pair.

The relation forbids both being false while allowing both true.

Statement 2 is the particular affirmative, Statement 4 the particular negative.

On the square of opposition the two particular propositions are sub-contraries.

At least one of them must be true, so they cannot both be false.

This is exactly the sub-contrary relation the question demands.

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