Ionizing radiation, benzene, and certain viruses can each individually cause leukemia without the other two exposures, but only if the exposed individual has a certain genetic deficiency. This genetic deficiency by itself does not cause leukemia, but the disease never occurs in the absence of this genetic factor. The genetic deficiency is

A necessary but not sufficient cause

Explanation:

A necessary but not sufficient cause is defined as any cause/condition that is an obligate requirement for a particular event to occur (in this case, the particular event is leukemia disease), but it isn't sufficient for the occurrence of this event. Conversely, a sufficient cause is defined as any cause/condition that will cause by itself the event to occur. Finally, a condition is neither necessary nor sufficient when the condition neither is required nor sufficient for the event to occur.

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