RC 2.146. The primary purpose of the passage is to
A. trace the development of a particular behavioral characteristic of the sloth bear
B. explore possible explanations for a particular behavioral characteristic of the sloth bear
C. compare the defensive strategies of sloth bear cubs to the defensive strategies of cubs of other
bear species
D. describe how certain behavioral characteristics of the sloth bear differ from those of other
myrmecophagous mammals
E. provide an alternative to a generally accepted explanation of a particular behavioral
characteristic of myrmecophagous mammals
此题我选了E,因为我觉得文章的主要对象是myrmecophagous mammals而不仅仅是the sloth bear。这一类提及了一种动物同时还提及其中的几种特例的文章如何确定文章的主要对象？
原文：
RC 2.145 – 2.148
The sloth bear, an insect-eating animal native to Nepal, exhibits only one behavior that is truly
distinct from that of other bear species: the females carry their cubs (at least part-time) until the
cubs are about nine months old, even though the cubs can walk on their own at six months.
Cub-carrying also occurs among some other myrmecophagous (ant-eating) mammals; therefore,
one explanation is that cub-carrying is necessitated by myrmecophagy, since myrmecophagy entails
a low metabolic rate and high energy expenditure in walking between food patches. However,
although polar bears’ locomotion is similarly inefficient, polar bear cubs walk along with their
mother. Furthermore, the daily movements of sloth bears and American black bears which are
similar in size to sloth bears and have similar-sized home ranges reveal similar travel rates and
distances, suggesting that if black bear cubs are able to keep up with their mother, so too should
sloth bear cubs.
An alternative explanation is defense from predation. Black bear cubs use trees for defense,
whereas brown bears and polar bears, which regularly inhabit treeless environments, rely on
aggression to protect their cubs. Like brown bears and polar bears (and unlike other
myrmecophagous mammals, which are noted for their passivity), sloth bears are easily provoked to
aggression. Sloth bears also have relatively large canine teeth, which appear to be more functional
for fighting than for foraging. Like brown bears and polar bears, sloth bears may have evolved in
an environment with few trees. They are especially attracted to food-rich grasslands; although few
grasslands persist today on the Indian subcontinent, this type of habitat was once wide spread there.
Grasslands support high densities of tigers, which fight and sometimes kill sloth bears; sloth bears
also coexist with and have been killed by tree-climbing leopards, and are often confronted and
chased by rhinoceroses and elephants, which can topple trees. Collectively these factors probably
selected against tree-climbing as a defensive strategy for sloth bear cubs. Because sloth bears are
smaller than brown and polar bears and are under greater threat from dangerous animals, they may
have adopted the extra precaution of carrying their cubs. Although cub-carrying may also be
adoptive for myrmecophagous foraging, the behavior of sloth bear cubs, which climb on their
mother’s back at the first sign of danger, suggests that predation was a key stimulus.
我的解答：
