Causal relationships exist when one event produces another. All causes are correlations, but not all correlations are causes. A sufficient cause is one that, all by itself, is enough to produce a certain result. A necessary cause is one that is required to produce a given effect, even if it's not sufficient by itself to bring it about. A contributory cause is one that makes a certain result more likely to happen.
Icy roads are a contributory cause of traffic accidents.
They are not a sufficient cause because sometimes the roads are icy and there are no traffic accidents. They are not a necessary cause because there can be traffic accidents when the roads are not icy. Given proposition B, homocysteine could not be a sufficient or necessary cause of heart attacks and strokes. At most, it would be a contributory cause. It's not a sufficient cause because some people have high levels and do not get heart attacks or strokes, and it's not necessary because some people do not have high levels and still get heart attacks or strokes.