单选题 .  ①Because different mammalian species favor different environments, identifying and counting bones from prehistoric deposits in caves can reveal much about climatic changes. ②However, using large mammals' bones can be problematic. ③Some species, such as red deer, are very adaptable—at home both on open grassland and in thick woodland. ④Moreover, some large-animal bones may have traveled considerable distances before being discarded: both carnivores and humans can have large hunting territories and bring home large animals quite unlike those near their den or campsite. ⑤Consequently, the bones of the small mammals found within cave sediments provide a better index of climate change: they are generally more numerous, the species are more sensitive to environmental conditions, and few travel far within their short lives.39.  The passage mentions which of the following as an obstacle to using animal bones to reconstruct past climate changes? ______