Scientists from the Max Plank Institute for Biological Cybernetics in Tubingen, Germany, have presented the first experimental evidence that people do end up walking in circles if lost in unfamiliar areas．The study, published in the journal Current Biology, examined the tracks of people who walked for several hours in the Sahara desert in Tunisia and in the Bienwald forest in Germany．Researchers Jan Souman and Marc Ernst said the scientists used the global positioning system （GPS） to record these paths．The results showed that the walkers were only able to keep a straight line when the sun or moon was visible．As soon as the sun disappeared behind some clouds, people started to walk in circles without even noticing it．
Souman said one explanation offered in the past for people walking in circles was that most people have one leg longer or stronger than the other, which would produce a systematic bias（偏差） in one direction．To test this．the researchers asked people to walk straight while blindfolded which removed the effects of vision．They found that these circles were rarely in a systematic direction, with the same person sometimes switching to the left and sometimes to the right．
"Most of the participants in the study walked in circles, sometimes in extremely small ones．Walking in circles is therefore not caused by differences in leg length or strength, but more likely the result of increasing uncertainty about where straight ahead is," said Souman, "Small random errors （随机性误差） in the various sensory signals that provide information about walking direction add up over time, making what a person perceives to be straight ahead drift away from the true straight ahead direction．"
72．The underlined word "this" in Paragraph 2 refers to _____．
A．the effects of vision             B．a systematic bias
C．the leg length differences         D．one explanation offered in the past
73．What is the probable reason for people's walking in circles?
A．The invisible sun or moon．
B．The increasing uncertainty about directions．
C．Differences in leg length or strength．
D．Wrong signals providing information about areas．
74．Why are the subjects required to be blindfolded?
A．To encourage them to walk straight．    B．To stop them from seeing each other．
C．To dismiss the effects of vision． D．To keep the test a top secret．
75．Which of the following statements is true?
A．People walk straight in the day．
B．The farther people walk, the greater the direction bias grows．
C．People walk only in large circles．
D．Researchers use the global positioning system to record their tracks．