Which of the following statements are so related that if one of them is true the other must be false and vice-versa?

1.All human beings are mortal

2.Some human beings are mortal

3.Some human beings are not mortal

4.No human beings are mortal

A1 & 3 only ✓ Correct

B2 & 3 only

C3 & 4 only

D1 & 4 only

Correct answer: (A) 1 & 3 only

Explanation

Statements 1 and 3 turn opposite in truth: one true forces the other false.

Statement 1 "All human beings are mortal" is an A proposition.

Statement 3 "Some human beings are not mortal" is an O proposition.

A and O are contradictories on the square of opposition.

Contradictories cannot share a truth-value, so each true makes the other false.

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