1. Txoj kev ntim cov khoom ci kom paub meej tias cov khoom ci ci nyob twj ywm tshiab thiab qab thaum lub sijhawm thauj mus los, kev ntim khoom kom zoo yog qhov tseem ceeb.Ua ntej, xaiv cov khoom ntim cov ntaub ntawv, xws li cov ntawv roj, cov khoom ntim cov hnab yas thiab cov yeeb yaj kiab npuas, kom tiv thaiv cov khoom ya raws, deterioration, lossis ...
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