A、They look true blue underwater. B、They look blue on the surface. C、They have blue underbellies. D、They have blue microorganism

Whales are huge, but difficult to see, which adds to their mystery and fascination. They are highly intelligent animals with an elaborate social life, no possessions and the complete freedom of movement in three dimensions.
     (23)Blue Whales are difficult to weigh because of their size, nevertheless, at up to 200 tons, even an " ordinary" sized whale is a vast and impressive creature. Their tongues can weigh as much as an elephant. Their heart weighs as much as an automobile, quite a size!
     (24)As their names suggested, blue whales look true blue underwater, but on the surface their coloring is more like blue-gray. Their underbellies take on a yellowish color from the millions of microorganisms that take up residence in their skin.
    Blue whales live in all the world’s oceans, occasionally swimming in small groups but usually alone or in pairs. They often spend summers feeding in polar waters and undertake lengthy migrations towards the Equator as winter arrives. These graceful swimmers cruise the ocean at more than five miles an hour, but accelerate to more than 20 miles an hour when they are angry. Blue whales are among the loudest animals on the planet.(25)They emit a series of pulses, groans, and moans, and it’s thought that, in good conditions, blue whales can hear each other up to 1, 000 miles away. Scientists think they use these methods not only to communicate, but, along with their excellent hearing, to navigate the lightless ocean depths.
23. Why are blue whales difficult to weigh?
24. Why do we call the animals blue whales?
25. Why can blue whales communicate with each other in a long distance?