求分析一道GWD阅读~~目的题

ljyjulia

GWD-TN-12
Until recently, zoologists believed that all species of phocids (true seals), a pin-nipped family, use a different maternal strategy than do otariids (fur seals and     sea lions), another pinniped family. Mother otariids use a foraging strategy. They acquire moderate energy stores in the form of blubber before arriving at breeding sites and then fast for 5 to 11 days after birth. Throughout the rest of the lactation (milk production) period, which lasts from 4 months to 3 years depending on the species, mother otariids alternately forage at sea, where they replenish their fat stores, and nurse their young at breeding sites. Zoologists had assumed that females of all phocids species, by contrast, use a fasting strategy in which mother phocids, having accumulated large energy stores before they arrive at breeding sites, fast throughout the entire lactation period, which lasts from 4 to 50 days depending on the species. However, recent studies on harbor seals, a phocids species, found that lactating females commenced foraging approximately 6 days after giving birth and on average made 7 foraging trips during the remainder of their 24-day lactation period.
The maternal strategy evolved by harbor seals may have to do with their small size and the large proportion of their fat stores depleted in lactation. Harbor seals are small compared with other phocids species such as grey seals, all of which are known to fast for the entire lactation period. Studies show that mother seals of these species use respectively 84 percent, 58 percent, and 33 percent of their fat stores during lactation. By comparison, harbor seals use 80 percent of their fat stores in just the first 19 days of lactation, even though they occasionally feed during this period. Since such a large proportion of their fat stores is exhausted despite feeding, mother harbor seals clearly cannot support all of lactation using only energy stored before giving birth. Though smaller than many other phocids, harbor seals are similar in size to most otariids. In addition, there is already some evidence suggesting that the ringed seal, a phocids species that is similar in size to the harbor seal, may also use a maternal foraging strategy.
The primary purpose of the passage is to D
A.    present evidence that several phocids species use the maternal fasting strategy
B.    explain why the maternal strategy typically used by phocids is different from the maternal strategy used by otariids
C.    argue that zoologists’ current understanding of harbor seals’ maternal strategy is incorrect
D.    describe an unexpected behavior observed in harbor seals and propose an explanation that may account for the behavior
E.    describe evidence concerning the maternal strategy of the harbor seal and suggest that the harbor seal belongs to the otariid rather than to the phocids family
这道目的题 答案为什么是D

straw520fish

until recently means before this time,if you check the line 20, you will find the answer is E, they use a fast strategy means they  don't replenish until they finished lactation.
You can find that the authoor used recent studies in the following  sentence.

Zoologists had assumed that females of all phocids species, by contrast, use a fasting strategy in which mother phocids, having accumulated large energy stores before they arrive at breeding sites, fast throughout the entire lactation period, which lasts from 4 to 50 days depending on the species. However, recent studies on harbor seals, a phocids species,