It is clear that humans during the Upper Paleolithic period used lamps for light in caves. Though lamps can be dated to the entire Upper Paleolithic, the distribution of known lamps from the period is skewed, with the greatest number being associated with the late Upper Paleolithic period, when the Magdalenian culture was dominant. Each of the following, if true, contributes to an explanation of the skewed distribution of lamps EXCEPT: ['Artifacts from early in the Upper Paleolithic period are harder to identify than those that originated later in the period.', 'More efficient lamp-making techniques were developed by the Magdalenian culture than by earlier cultures.', 'More kinds of lamps were produced by the Magdalenian culture than by earlier cultures.', 'Fire pits were much more common in caves early in the Upper Paleolithic period than they were later in that period.']