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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.
单项选择题
A．It has lost its primary oxygen sources.B．It has lost its backup oxygen sources.C．It has lost both the primary and the backup oxygen sources.D．It has lost neither the primary or the backup oxygen sources.
A．It has lost its primary oxygen sources.
B．It has lost its backup oxygen sources.
C．It has lost both the primary and the backup oxygen sources.
D．It has lost neither the primary or the backup oxygen sources.
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