Multiple choice

Which one of the following is not a valid C identifier?

  1. ___S

  2. 1___

  3. ___1

  5. S___
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

C identifier rules: must start with letter or underscore, can contain letters, digits, and underscores. '1___' starts with a digit, making it invalid. The other options all start with underscores and valid characters, which is allowed (though some underscore patterns are reserved for system use).

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