Numeric 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, numeric expressions only produce numeric values
  6. When evaluated, numeric expressions may produce numeric or string values
  7. Numeric expressions must have single values
  8. Numeric expressions may have single values
  9. Numeric expressions may contain operators
  10. Numeric expressions must contain operators
  11. Numeric expressions may contain numeric variables
  12. Numeric expressions must contain numeric variables
  13. Numeric expressions must not contain string variables
Explain your answer.

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