valid identifier in C language?

PRACTICE! Determine which of the following names are valid identifiers. If a name is not a valid identi fier, give the reason that it is not acceptable, and suggest a valid replacement. 1. density 4. xsum 7. perimeter 10. break 13. count 16. f2 19. referencel 2. area 5. x_sum 8. secA2 11. #123 14. void 17. Final Value 20. reference_1 3. Time 6. tax-rate 9. degrees 12. x&y 15. f(x) 18. w1.1 21. m/s