Question

If no birds are mammals, and some animals are birds, which of the following must be true?

• A. All animals are mammals.
• B. Some animals are not mammals.
• C. No animals are birds.
• D. All birds are animals.
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Hint:

In critical reasoning questions, use logical deductions or Venn diagrams to identify what must be true. Avoid options that are only possible or contradict the premises.

Answer 1

The Correct Option is B

- Step 1: Break down the premises. The question provides two premises:
- Premise 1: No birds are mammals (birds and mammals are disjoint sets, with no overlap).
- Premise 2: Some animals are birds (at least one animal is a bird).
- Step 2: Analyze logical implications. Since some animals are birds, and no birds are mammals, those animals that are birds cannot be mammals. This implies that there are animals (the birds) that are not mammals.
- Step 3: Evaluate the options.
- Option (1): "All animals are mammals" is incorrect, as the bird-animals are not mammals, contradicting the premise.
- Option (2): "Some animals are not mammals" is true, as the animals that are birds (from premise 2) are not mammals (from premise 1), making it a strong candidate.
- Option (3): "No animals are birds" directly contradicts the second premise, so it is incorrect.
- Option (4): "All birds are animals" is possible but not necessarily true, as the premises do not confirm that all birds are animals, only that some animals are birds.
- Step 4: Confirm with a Venn diagram. Imagine animals as a large circle, with birds as a smaller circle inside it (from premise 2). Birds are outside the mammals circle (from premise 1), confirming that some animals (birds) are not mammals.
- Step 5: Verify necessity. Option (2) is the only statement that must be true, as the existence of birds as animals ensures non-mammal animals.
- Step 6: Final conclusion. Option (2) Some animals are not mammals is the correct answer, as it logically follows from the premises.
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