28. Paleoanthropologists examine fossil (remains) of extinct primates, while physical anthropologists (concern) with ethology study the behavior of primates (in) their (natural settings). 选b,为什么?? 39. (By) 1810 the 23 towns of Hampshire County, Massachusetts, (had reached) a remarkable (uniform) of economic development (as well as) population density. 选d,为什么??