题目

阅读理解  There are many people on the bus. Some have seats, but some have to stand. At a bus stop, a woman gets on the bus. An old man tries to stand up. “Oh, no, thank you,” the woman asks him back to the seat. “Please don’t do that. I can stand.” “But, madam, let me…,” says the man. “I ask you to sit on your seat,” the woman says. She puts her hands on the old man’s shoulder. But the man still tries to stand up, “Madam, will you please let me…?” “Oh, no,” says the woman. She again asks the man to sit back to his seat. At last the old man shouts, 北方的冬天，在菜窖里放几桶水，菜就不易被冻坏．这是因为水在发生________（填物态变化名称）时要________热量，使窖内的温度不至于太低．