下列过程中发生了化学变化的是[ ]
A.用金刚石切割玻璃
B.用铅笔在纸上写字
C.用醋酸清洗热水瓶的水垢
D.用水将浓盐酸稀释
答案：C
After doing market research, all the researchers think it ________ to invest a large sum of money
in the project.[ ]
A. worthless
B. priceless
C. valuable
D. worthwhile
举例说明怎样对文件进行压缩和备份？
