The mistake of "well-off family" in item A generally describes people who are knowledgeable or have expertise in learning. This should mean generosity.

The "unimaginable" in item B can only be said as "the consequences will be unimaginable" and cannot be used as an attribute, so item B is wrong.

The "endless" mistakes in item D mostly describe critical situations or low voices and breathing, which are intermittent. It also means that skills are on the verge of being lost. This should be "endless"

Are you satisfied with this explanation?