Europeans have long known that eating quail sometimes makes the eater ill, but only recently has it been established that the illness is caused by a toxin present in the quail's body only under certain conditions.

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A Europeans have long known that eating quail sometimes makes
B Europeans have long known quail eating is sometimes able to make
C Eating quail has long been known to Europeans to sometimes make
D It has long been known to Europeans that quail eating will sometimes make
E It has long been known to Europeans that quail, when it is eaten, has sometimes made