String expressions

Which of the following is/are true?
  1. Numeric expressions may only contain numeric operators
  2. String expressions may only contain string operators
  3. Numeric expressions may only contain numeric operands
  4. String expressions may only contain string operands
  5. When evaluated, string expressions only produce string values
  6. When evaluated, string expressions may produce numeric or string values
  7. String expressions must have single values
  8. String expressions may have single values
  9. String expressions may contain operators
  10. String expressions must contain operators
  11. String expressions may contain string variables
  12. String expressions must contain string variables
  13. String expressions must not contain numeric variables
Explain your answer.

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