When defining a $1$:$1$ relationship where both sides of the relationship are mandatory, $\_\_\_\_\_.$

a$.$ the model should be revised to ensure that the two entities do not belong together

b$.$ the foreign key should be placed in both entities in the relationship

c$.$ the foreign key should be placed in the entity in which the relationship role in not played

d$.$ the foreign key that causes the fewest nulls should be selected