Which of the following schedules is cascadeless?

a$.$

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

b$.$

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

c$.$

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

d$.$

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