$ 16,800 is the collector's value of（）

单项选择题A postage stamp's value to collectors is raised if （）

A．there are few others like it
B．there are no errors on the stamps
C．a mistake is made in the ,printing
D．both A and C

单项选择题How long did the Knights of Labor last（）

A．60 years.
B．31 years.
C．50 years.
D．21 years.