(a) Yes, that is the best answer. It often happens that causes combine to produce particular harm. This seems to be such a case. The accident would not have occurred if A had driven more slowly and kept to his side of the line. But equally, it wouldn't have happened if B had kept a proper lookout and taken action to avoid the collision. So it is the combined negligence of A and B that produced the harm.
Cases of this sort used to be legally problematic for a plaintiff. In the first place, the 'but for' test does not give sensible results in such cases. And, in the past, a plaintiff could not claim damages if they had partly contributed to their own loss by being negligent.
Legislation has simplified the issue by allowing a court to divide (apportion) the responsibility for the harm, depending on the extent to which each party was responsible for causing it. The courts commonly apportion the blame by using percentages. If, in this case, A and B were equally to blame, they would each be liable for 50% of the harm. B could then claim half his loss from A, while having to bear the other half of the loss himself.