下列变化过程中，一定发生了化学变化的是（   ）

质量为3kg的物体放在高4m的平台上，g取10m/s²．求：

根据对话内容，从下面方框中选择适当的单词或词组填空，使对话内容完整正确。
medicine,      started,      terrible,      often,      wrong   
B: Bob                  D: Doctor
D: Come and sit down, Bob. What’s ____ with you?
B: I have a ____  pain in my stomach, doctor.
D: I see. When did it start?
B: It ____  yesterday. I didn’t want to eat anything. I’m not feeling well now.
D: Let me see, er, you have a fever.
B: I’ve got a headache, too. And my throat hurts.
D: Well, you’ve caught a bad cold.
B: I know these days the H7N9 flue is very popular. Is it serious?
D: No, not very serious, your temperature isn’t very high. Pay attention to your food and remember to wash your hands before meals. And you must stay in bed for 2 or 3 days and take the Chinese ____ .
B: How____ should I take it?
D: Three times a day after meals.
B: Thank you.

物理课上，老师把两根铅柱的端面削平，将削平的端面相对，并用力压紧，使两根铅柱合在一起．如图所示，在铅柱下面悬吊一定的重物，两根铅柱也不会分开，小华认为是大气压把两个铅柱压在一起．请你设计一个实验证明小华的观点是错误的．

请选出下列说法错误的一项（    ）