Expert systems are expensive and time consuming to maintain because:

Group of answer choices

A$.$ their rules must be reprogrammed every time there is a change in the environment, which in turn may change the applicable rules.

B$.$ their rule base is so complex.

C$.$ they rely on equipment that becomes outdated.

D$.$ it is difficult to program at this level of complexity without introducing software bugs.

E$.$ only the person who created the system knows exactly how it works, and may not be available when changes are needed.