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[多选题]31、闪光灯有哪几种闪光模式？
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9.A.sight B.sign C.insight D.slightness
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21.Video recorders and photocopiers, even ticket machines on the railways, often seem unnecessarily difficult to use. Last December I bought myself a Video cassette recorder (VCR) described as “simple to use”. In the first three weeks I failed repeatedly to program the machine to record from the TV, and after months of practice I still made mistakes. I am not alone. According to a survey last year by Ferguson, the British manufacturer, more than one in four VCR owners never use the timer on their machines to record a programme: they don’t use it because they’ve found it far too hard to operate.So why do manufacturers keep on designing and producing VCRS that are awkward to use if the problems are so obvious?First, the problems we notice are not obvious to technically minded designers with years of experience and trained to understand how appliances work. Secondly, designers tend to add one or two features at a time to each model, whereas you or I face all a machine’s features at once. Thirdly, although find problems in a finished product is easily, it is too late by then to do anything about the design. Finally, if manufacturers can get away with selling products that are difficult to use it, it is not worth the effort of any one of them to make improvements.Some manufacturers say they concentrate on providing a wide range of features rather than on making the machines easy to use. But that gives rise to the question, “why can’t you have features that are easy to use?” The answer is you can.Good design practice is a mixture of specific procedures and general principles. For a start, designers should build an original model of the machine and try it out on typical members of the public-not on colleagues in the development laboratory. Simple pubic trials would quickly reveal many design mistakes. In an ideal world, there would be some ways of controlling quality such as that the VCR must be redesigned repeatedly until, say, 90 percent of users can work 90 per cent of the features correctly 90 per cent of the time.(10分)21.1 The author had trouble operating his VCR because ________.(2分)A. he had neglected the importance of using the timerB. the machine had far more technical features than necessaryC. he had set about using it without proper trainingD. its operation was far more difficult than the designer intended it to be
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( )是指由谁推动一个社会的政治发展问题。
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胃蠕动波的频率为每分钟
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14.在学生关系中设定学生“成绩”值在0~100之间，这种约束属于关系模型的
