单项选择题
A．their dorsal fins are very valuable B．they have a swim bladder C．they have fins for swimming D．they have skeletons made of cartilage
A．humans' friends B．man's enemies C．the world's largest fish D．not the same as other fish
A．more than one million sharks are killed by humans every year B．there are fewer than one hundred shark attacks on humans in the world each year C．many people are afraid of sharks D．most sharks eat fish and seals and other sharks