Short explanation answer for each of the following.

1. The elements of negligence,
2. contributory negligence,
3. comparative negligence,
4. assumption of the risk,
5. valid contract,
6. void contract,
7. voidable contract,
8. unenforceable contract,
9. elements of a contract,
10. condition concurrent,
11. condition precedent,
12. condition subsequent,
13. rejection,
14. Revocation
15. ratification,
16. disaffirmance and in pari delicto.