Which of the following statements describes fan traps in ERD design?

a$.$ A fan trap occurs when there is one entity in two $1$:M relationships to other entities.

b$.$ A fan trap occurs when there is one entity in two $1$:M relationships.

c$.$ A fan trap occurs when there is one entity in two $1$:$1$ relationships to other entities.

d$.$ A fan trap occurs when there is one entity in two $1$:$1$ relationships.