Real the following passage and mark the letter A, B, C, or D on your answer sheet to indicate the correct word or phrase that best fits each of the numbered blanks from 18 to 22.

Can robots advance so far that they become the ultimate threat to our existence? (18) _______ . The human brain, they argue, is the most complicated system ever created, and any machine designed to reproduce human thought is bound to fail. Physicist Roger Penrose of Oxford University and others believe (19) _______ . Colin McGinn of Rutgers University backs this up when he says that Artificial Intelligence is like sheep trying to do complicated psychoanalysis. (20) _______ .
Artificial Intelligence, or Al, is different from most technologies in that scientists still understand very little about how intelligence works. Physicists have a good understanding of Newtonian mechanics and the quantum theory of atoms and molecules, whereas the basic laws of intelligence remain a mystery. But a sizeable number of mathematicians and computer scientists, who are specialists in the area, are optimistic about the possibilities. (21) _______ . Over the years, various problems have impeded all efforts to create robots. To attack these difficulties, researchers tried to use the 'top-down approach', using a computer in an attempt to program all the essential rules onto a single disc. (22) _______ .