单项选择题

Questions 29 to 32 are based on the passage you have just heard.

A．Four years.
B．Five years.
C．Three years.
D．Six years.
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单项选择题
C
单项选择题
______is particularly harmful to photographic materials.
A．Cignin
B．Bleach
C．Acid
D．Chemical whiteners
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