Whales are big，but why aren't they even bigger？A new study says the key factor is how many calories they can take in.Researchers came to this conclusion after using small boats to chase down 300 whales of various species around the world.They reached out with long poles to attach sensors（傳感器）to the creatures，allowing them to record what whales were doing as they dived for food.
   The study included 90 blue whales，the biggest animals ever to live.Size estimates vary，but they can grow to about 100 feet long and weigh more than 100 tons，according to the American Cetacean Society.The research also included sperm whales，the biggest whales with teeth；they can grow to about 60 feet in length.
   Toothed whales hunt their prey（獵物）one at a time，seeking them out in the dark depths with a sonar-like detection.But as their body size increases，their hunting style becomes progressively less efficient - they take in fewer calories from their meals than the number they burn in each dive.That's a result of the limited availability of prey.
   Blue whales grow bigger because they feed in a much different and more efficient way，the researchers reported.They open their mouths wide to take in an enormous volume of water - even bigger than their body size - as they swim along.They then close their mouths and strain the water out through a sieve-like filter（過濾器），trapping huge numbers of tiny shrimp-like creatures called krill.That "allows them to take advantage of a very large prey resource that's very rich in a small amount of time，" said Jeremy Goldbogen of Stanford University，another member of the research team. "They get a huge bang for the buck."
   So why aren't they even bigger than they are？"Perhaps blue whales are limited by the seasonal nature of the most nutritious krill patches，which grow huge and dense in the summer，" Goldbogen said. "And calculations indicate that to sustain a blue whale that's 115 feet long，you'd need a density of prey that is just not seen anywhere in the world."

（1）How did researchers conduct the study？

B

B

A．By detecting whales with poles.
B．By tracking whales with devices.
C．By filming whales' preying on boats.
D．By recording whales' sound from sensors.
（2）What do we know about toothed whales？

D

D

A．They can grow to about 100 feet long.
B．They seek prey in the shallow waters.
C．They hunt less efficiently as they grow older.
D．They dive consuming more calories as they grow bigger.
（3）What makes blue whales different from toothed whales in size？

D

D

A．Blue whales have big mouths.
B．Blue whales hunt in the deep sea.
C．Blue whales eat more varieties of foods.
D．Blue whales hunt for food more efficiently.
（4）What does the underlined sentence in paragraph 4 refer to？

C

C

A．Preying takes more time than before.
B．Preying becomes easier than expected.
C．Whales get a greater harvest from preying.
D．Whales consume more calories than they obtain.