Which XXX completes the Python partition$()$ function used by quicksort?

$$def partition$($numbers$,$ start$\_$index, end$\_$index$)$: midpoint $=$ start$\_$index $+($end$\_$index $-$ start$\_$index$)//2$ pivot $=$ numbers$[$midpoint$]$ low $=$ start$\_$index high $=$ end$\_$index done $=$ False while not done: while numbers$[$low$]<$ pivot: low $=$ low $+1$ while pivot $<$ numbers$[$high$]$: high $=$ high $-1$ XXX: done $=$ True else: temp $=$ numbers$[$low$]$ numbers$[$low$]=$ numbers$[$high$]$ numbers$[$high$]=$ temp low $=$ low $+1$ high $=$ high $-1$ return high

a$)$ if numbers$[$low$]>$ numbers$[$high$]$

b$)$ if numbers$[$low$]<$ numbers$[$high$]$

c$)$if low $>=$ high

d$)$ if low $<$ high