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.
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