Which of the following schedules is non$-$recoverable?

a$.$

Sch$4$: r $1($X$)$; w $1($X$)$; r $1($Y$)$; r $2($X$)$; w $1($Y$)$; C $1$ ; w $2($X$)$; C $2$ ;

b$.$

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

c$.$

Sch$3$: r $1($X$)$; w $1($X$)$; r $1($Y$)$; w $1($Y$)$; r $2($X$)$; w $2($X$)$; C $2$ ; C $1$ ;

d$.$

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