Pickup the correct statement:

Statement: AVL imbalance before deletion

$P$:$R_0$ balance factor of ancestor $=-1$

$Q$:$R_1$ balance factor of ancestor $=-1$

$R$:$R_{-1}$ balance factor of ancestor $=-1$

$S$:$L_0$ balance factor of ancestor $=-1$

$($a$)P$

$($b$)$ Q

$($c$)R$

$($d$)S$