题目内容：
根据以下材料，回答题
HowWeRead
Nowletuslookathowweread．Whenwereadaprintedtext,oureyesmoveacrossapageinshort,jerkymovement．Werecognizewordsusuallywhenoureyesfixate．Eachtimetheyfixate,weseeagroupofwords．Thisisknownastherecognitionspanorthevisualspan．Thelengthoftimeofwhichtheeyesstop--thedurationofthefixation--variesconsiderablyfrompersontoperson．Italsovarieswithinanyonepersonaccordingtohispurposeofreadingandhisfamiliaritywiththetext.
Furthermore,itcanbeaffectedbysuchfactorsaslightingandtiredness.
Unfortunately,inthepast,manyreadingimprovementcourseshaveconcentratedtoomuchonhowoureyesmoveacrosstheprintedpage．Asaresultofthismisleadingemphasisonthepurelyvisualaspectsofreading,numerousexerciseshavebeendevisedtotraintheeyestoseemorewordsatonefixation．Forinstance,insomeexercises,wordsareflashedontoascreenfor,say,atenthoratwentiethofasecond．Oneoftheexerciseshasrequiredstudentstofixtheireyesonsomecentralpoint,takinginthewordsoneitherside．Suchwordpatternsareoftenconstructedintheshapeofrathersteeppyramidssothereadertakesinmoreandmorewordsateachsuccessivefixation．Alltheseexercisesareveryclever,butit'sonethingtoimproveaperson'sabilitytoseewordsandquiteanotherthingtoimprovehisabilitytoreadatextefficiently．Readingrequirestheabilitytounderstandtherelationshipbetweenwords．Consequently,forthesereasons,manyexpertshavenowbeguntoquestiontheusefulnessofeyetraining,especiallysinceanyapproachwhich
trainsapersontoreadisolatedwordsandphraseswouldseemunlikelytohelphiminreadingacontinuoustext.
Thetimeoftherecognitionspancanbeaffectedbythefollowingfactsexcept__________. A．one'sfamiliaritywiththetext
B．one'spurposeofreading
C．thelengthofagroupofwords
D．lightingandtiredness
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答案解析：