题干
It's good manners to be ________________ while watching the raising of our national flag.
A：happy
B：serious
C：kind
D：generous
答案(点此获取答案解析）
B
It's good manners to be ________________ while watching the raising of our national flag.
A：happy
B：serious
C：kind
D：generous
精确到＿＿＿＿位，有效数字是＿＿