Read the following passage and mark the letter A, B, C or D on your answer sheet to indicate the correct word that best fits each of the numbered blanks from 27-38.
DISAPPEARING ALASKAN SEALS
On a winter afternoon, a dozen male Northern fur seals are on an island off south-western Alaska. To look at them, fighting for territory on the rocks, it’s hard to imagine that they are at the (27)____________of a baffling scientific mystery: why is this species beginning to disappear? These seals- which can weigh up to 270 kilograms-have an important and symbolic (28)_________in Alaska’s history. Their soft and luxurious fur, the coveted prize during the huge commercial sea harvests that were legal here in the Bering Sea through the early 20th century, was once so valuable it (29)_________Alaska’s economy. Starting in the 1950s, for reasons that are (30)_______because the harvests by then tended to be fairly small, the seal population began a slow decline . But lately it has been falling drastically, declining at an alarming (31)________of 6 per cent a year since 1998. From a (32)_________ of more than 2 million in Alaska in 1948, their numbers have fallen to about 1 million.
So (33)________, no one has been able to establish a precise cause for this, although theories (34)___________. The seals' food supply may be (35)________, or another species may be (36)________on the seals- perhaps killer whales, which no longer have as many great whales to eat because of harvesting of those mammals. Some people have suggested that the decline can be (37)______to entanglement in fishing nets ,but scientists say they doubt that this alone could (38)_________the recent population decline.
Điền vào ô 33.
A. long
B. far
C. on
D. forth
Đáp án B
So far: cho đến nay
So (33), no one has been able to establish a precise cause for this: cho đến nay không ai có thể lập luận được 1 lí do chính đáng cho việc này.