Why Are Two Incubation Sessions Needed in the Donath-Landsteiner Test

Why are two incubation sessions needed in the Donath-Landsteiner test?

A) To detect the biphasic thermal reactivity of the offensive antibody
B) To detect complement binding in vivo
C) To detect the antibody's specificity to the P antigen in vitro
D) To detect the antibody's specificity to group O cells