题目

P为矩形ABCD所在平面外一点，且PA⊥平面ABCD，P到B，C，D三点的距离分别是，，，则P到A点的距离是                。                  答案：1American researchers say drinking tea may help strengthen the body’s defense system against infection（感染）. Doctors at Brigham and Women’s Hospital in Boston, Massachusetts, did the study. The team studied a chemical found in black, green, oolong and pekoe tea. This chemical is an amino acid（氨基酸）called L-theanine. The scientists say it may increase the strength of Gamma delta T cells, which are part of the body’s defenses.  First, the researchers mixed some of these cells with antigens（抗原）found in the amino acid. Antigens help the body react to infection. Then the scientists added some bacteria（细菌）. Within twenty-four hours, the cells produced a lot of interferon, a substance that fights infection. Cells not mixed with the antigens did not produce interferon.  In the second part of the study, eleven people drank five to six cups of black tea every day. Ten other people drank the same amount of instant coffee. That is dried coffee mixed with hot water.  Two weeks later, and again two weeks after that, the researchers tested the blood of all twenty-one people. They also looked at what happened when they added bacteria to the blood cells. They found that the tea drinkers produced five times more interferon after they started drinking tea. The coffee drinkers did not produce interferon.  Doctor Jack Bukowski led the study. He says the antigens added to the Gamma delta T cells were responsible for the increased reaction to the bacteria. He says the study also showed that the cells were able to remember the bacteria and fight them again the next time.  Earlier research already has found that tea can help prevent heart disease and cancer. Doctor Bukowski says the new study must be repeated with more people. If the findings prove to be true, he says, then tea drinking might also help protect against bacterial infections. He says the amino acid L-theanine could be removed from tea and used as a drug to strengthen the body’s defenses.  67．We may know from the text that _____ can be found in different kinds of tea.  A．Gamma delta T cells     B．L-theanine       C．interferon          D．bacteria  68．Tea may help strengthen the body’s defense system because it helps _____.  A．the body to produce more interferon  B．the body to produce more Gamma delta T cells  C．to add some bacteria to the blood cells  D．to mix antigens with some of the cells in the body  69．According to Dr Bukowski, _____.  A．the findings of the study have already proved to be true  B．he has taken some amino acid L-theanine from tea and made a drug with it  C．further study is needed to prove the findings true  D．he is not sure whether tea can help prevent heart disease and cancer  70．What would be the best title for this text?  A．Tea Is Better Than Coffee.             B．Tea May Help Fight Infection  C．Tea Can Help Prevent Cancer.         D．Our Body Needs Tea