In Common Law,contracts That Unduly Interfere with the Ability to Earn

In common law,contracts that unduly interfere with the ability to earn a livelihood or reduce competition are generally unenforceable.Why is this so?

A) They contravene the rules of good will contained in the Competition Act.
B) They cannot be enforced because they are contrary to statues and public policy.
C) They contravene the rules of fairness contained in the Consumer Protection Act.
D) They are contrary to the community's common sense and common conscience.