Critical reasoning — assumptions (implicit): Statement: Today I must satisfy myself only by looking at a pink-headed duck in an encyclopaedia. Assumptions: I. Pink-headed ducks are as good as extinct now. II. People refer to an encyclopaedia only to know about things which do not exist now.

Introduction / Context:
The speaker says they must be content with merely viewing a pink-headed duck in an encyclopaedia today. This suggests that seeing a real specimen is not feasible at present. We must determine which assumptions are required by this remark.

Given Data / Assumptions:

• Desire to see a pink-headed duck, but only possible in a reference book today.
• Assumption I: The species is practically extinct or at least not observable.
• Assumption II: Encyclopaedias are used only for non-existent things.

Concept / Approach:
For the statement to make sense, it must be hard or impossible to see the bird in real life now. That is captured by Assumption I. The second assumption is overly strong and false; people consult encyclopaedias for existing and non-existing things alike, so II is not required.

Step-by-Step Solution:

Verification / Alternative check:
Negate I: If pink-headed ducks were easily observable today, the speaker would not need to rely on an encyclopaedia, undercutting the statement. Negate II: Encyclopaedias can be used for many purposes; the original statement still makes sense. Thus I is required; II is not.

Why Other Options Are Wrong:

• Only II/Either/Neither/Both: Each either excludes the necessary I or adds an unnecessary II.

Common Pitfalls:
Interpreting 'look in an encyclopaedia' as implying that the subject no longer exists. The correct reading is about present inaccessibility, not an encyclopaedia's exclusive scope.

Final Answer:
Only assumption I is implicit