ISTQB Doubtful and Confusing Questions

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Question No.1.
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For a mandatory input field "ZIP code” the following rules are given
1. The valid ZIP code format is 5 numeric digits
2. The code has to exist in the post office's official ZIP code list
Using equivalence classes partitioning, how many test cases are required to test this field?

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Question No.2.
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An input screen accepts 3 inputs; each of these has 3 equivalence classes.
How many test cases are required for testing all combinations of equivalence classes?

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Question No.3.
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Which of the following BEST matches the attributes with a level of testing?
I. Stubs and drivers are often used
II. The test environment should correspond to the production environment
III. Finding defects is not the main focus
IV. Testing can be based on use cases
V. Testing is normally performed by testers
VI. Testing for functional and non-functional characteristics