After discovering that their software had a flaw, PDG issued "patches". Two years later, some PDG users were still suffering from the original flaw. Why?

a$.$

The website had copied PDG from somebody else and did not receive the PDG patches.

b$.$

Some users backed up the original PDG$.$ When informed of the patch, the users applied the patch. But there was an issue. The original backup was used to restore the system.

c$.$

all of the above

d$.$

People are lazy and do not update their software.