John and Jane are planning a family, but since each has a brother who has sickle-cell disease, they are concerned that their children may develop sickle-cell disease. Neither John, Jane, nor their respective parents have the disease. They consult a genetic counselor who tells them that .
a) there is no chance that any of their children will have sickle-cell disease
b) all of their children will have sickle-cell disease
c) one of four of their children can be expected to have sickle-cell disease
d) it's possible that none of their children will have the disease, but blood tests on them both will be required to make sure
e) it is impossible to tell whether their children will have the disease