在下列各句的每个空格内填上一个适当的、完整的单词，该词的首字母已给。

1. He's unhappy because of his little dog's d__________.

2. The reason why he was away was not m__________ in the text.

3. We must keep the s                  between you and me. Don't let others know them.

4. Paris is one of the l__________ cities in Europe.

5. Students are often a__________ while having classes in the hot afternoon

以铁铵钒为指示剂，用硫氰酸铵标准溶液滴定银离子时，应在下列何种条件下进行( )

A．酸性

B．弱酸性

C．碱性

D．弱碱性