(a) That's wrong. A duty of care is not breached as soon as a situation arises which may cause foreseeable harm. A person who owes a duty of care is only required to take appropriate steps to avoid the harm from occurring. The standard is: what steps would an ordinary, prudent person take in the circumstances to avoid the harm? To answer this question, a court will take account of various factors, including the following:
How likely was the harm? There is no need to guard against very remote possibilities.
How great would the harm be? You must guard more carefully against the risk of very great harm.
How difficult is it to avoid the harm? You need not adopt impractical measures.
Do the circumstances justify risking the harm, for example, if the steps needed to avoid the harm also pose a risk of harm?
Do policy considerations excuse the harm in question?
In the present case it is obvious that there were various things that could have been done to prevent the harm from occurring. A could have closed his gate. He could have tidied up his tools and moved the wheelbarrow to a safer place. He could have asked someone else to keep an eye on things while he was inside the house. One or more of these things was easy enough to do, and would have much reduced the risk of harm from occurring. They are the things a reasonable person would have done, and A should have done something along these lines. By simply leaving the equipment unattended in his front garden with the gate open, A has breached the duty of care that he owed.