题目内容：

Blue whales are the largest animals ever known to have existed on the planet, and they can grow to a length of 100 feet and weigh more than 330,000 pounds. But recently researchers have found that these whales are on the move and they have migrated (moved) from California waters to areas off Canada and Alaska for the first time since commercial whaling ended in 1965.

The researchers identified 15 blue whales that have appeared off the coast of British Columbia and the Gulf of Alaska since 1997. Four of these whales were recognized as ones that were once sighted off California shores, suggesting that the whales are returning to an old migration pattern between the coasts.

Before commercial whaling began in the early 1900s, blue whales were found widely throughout the North Pacific and California waters. But from the 1920s to the 1960s, whaling severely reduced the whale populations. Blue whales never recovered in the Northern Pacific, making sightings in this area rare. However, much larger groups of whales have been observed close to California since the 1970s.

The scientists had previously thought that the California population was separate from the population that had historically lived in North Pacific waters. But the current study shows that whales off the coast of British Columbia and the Gulf of Alaska are likely part of the California population.

To identify the blue whales, the researchers looked at photos of cetaceans taken in the North Pacific Ocean and compared them with a library of blue whale pictures taken along the West Coast of the United States and Southern Pacific. Up to now, the researchers are not quite sure why the whales are changing their migration patterns, but they suspect that the whales may be following their food moving farther north by the changes in ocean conditions.

1.According to the passage, blue whales are __________.

A. following the fixed route of migration

B. dying out due to commercial whaling

C. rarely found in the Southern Pacific

D. moving from the south to the north

2.The whale population in the northern Pacific __________.

A. is different from the California population

B. has become the largest group since the 1970s

C. is probably part of the California population

D. has lived there since the 1960s.

3.The underlined word “cetaceans” in Paragraph 5 probably means __________.

A. whales species       B. land creatures

C. ocean conditions     D. migration patterns

4.What might make blue whales migrate according to the researchers?

A. Commercial whaling.     B. Food resources.

C. Weather conditions.     D. Life reproduction.