(a) That's probably wrong. Liability in Negligence only exists if the harm suffered by the plaintiff can be sufficiently attributed to the wrongdoer's breach of their duty of care. In other words, it must be established that the wrongdoers conduct was the legal cause of the harm for which the plaintiff is claiming compensation. The common law asks: "Would the harm have occurred but for the defendant's conduct?" State and territory legislation now sets out similar principles.
To determine whether negligence caused particular harm it should be asked: (a) was the negligence a necessary condition of the occurrence of the harm? and (b) is it appropriate for the scope of the negligent person's liability to extend to the harm so caused? The plaintiff has the onus of proving, on the balance of probabilities, any fact relevant to the issue of causation.
In the present case, it is fairly clear that C's loss of income for the two days that he was without power is the direct result of A's negligence. But for A's negligence, C's power supply would not have been interrupted and his work delayed. The injury to C's back is not so closely connected to A's negligence. Although it could be argued that C would not have played golf if he had been able to continue his work, and that therefore his injury would not have occurred but for the cutting of the powerline, it is likely that this harm would be considered too remotely connected to A's negligence.