Which of the following schedules is serial?

a$.$

r $2($X$)$; w $2($X$)$; r $1($X$)$; w $1($X$)$; r $1($Y$)$; w $1($Y$)$;

b$.$

r $1($X$)$; w $1($X$)$; r $2($X$)$; r $1($Y$)$; w $1($Y$)$; w $2($X$)$;

c$.$

r $1($X$)$; w $1($X$)$; r $1($Y$)$; r $2($X$)$; w $2($X$)$; w $1($Y$)$;

d$.$

r $1($X$)$; r $2($X$)$; w $1($X$)$; r $1($Y$)$; w $1($Y$)$; w $2($X$)$;