Tina was a ten-year-old girl from England. Two weeks before the 2004 Christmas holiday, her teacher was talking about tsunamis（海啸） in a geography（地理） class at school. This helped her save lives on December 26th, 2004.On that morning, Tina and her family were walking on the Beach and enjoying the sun. Then suddenly, the water went out very far. It was bubbling, too. She told her mother at once. Her mum didn't understand. Tina was very sad. She shouted to her dad. He warned others and everyone left the beach. Because of her warning, the hotel workers quickly helped 100 guests get to a safer place.

—Is she eating?

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对密度定义式ρ=$\frac{\mathrm{m}}{V}$​的下列几种理解中，正确的是（　　）