Expert systems are expensive and time-consuming to
a. because their rule base is so complex.
b. because they rely on equipment that becomes outdated.
c. because their rules must be reprogrammed every time there is a change in the environment, which in turn may change the applicable rules.
d. because only the person who created the system knows exactly how it works, and may not be available when changes are needed.

Option C

Explanation: In simple words, expert systems refers to the software of artificial intelligence that helps humans in decision making by solving complex problems through using if- then rules rather than  conventional methods.

These software are used by entities such as financial and technical research organisations and the problems it solves highly depends on rapidly changing environmental factors , thus, a change in environment would demand a change in the system which cost heavy money to the entities using it.

answer; ///i believe that the correct answer is (a) it is the means by which a person earns money;