单项选择题

A．It has lost its primary 6xygen sources.
B．It has lost its backup oxygen sources.
C．It has lost both the primary and the backup oxygen sources.
D．It has last neither the primary or the backup oxygen sources.
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单项选择题
A．More than two billion.B．More than three billion.C．More than Four billion.D．More than Six billion.
A．More than two billion.
B．More than three billion.
C．More than Four billion.
D．More than Six billion.
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