物质

a

b

c

d

反应前质量（g）

10

10

10

2

反应后质量（g）

待测

2

21

2

Wu Ling: It's cool and sunny most of the time in our country．I live in Beijing, a beautiful city．It's July now．Though it is a little hot, I still feel comfortable．

Waxili: I live in Moscow．There are storms too often there．When the sky is dark and clouds are moving across the sky, the storm is coming．I don't like that．Always it's not safe in the storm．

John: I live in the north of Canada．It's quite cold in winter, so we wear warm clothes．Many people like snowy weather because they can make snowmen and enjoy themselves．

Joe: I come from Australia．I never see a snowy day．It's very hot most of the time in our country．In winter it's not very cold．