单项选择题

It can be inferred from the passage that before 1910 the normal running time of a film was ______.

A．15 minutes or less
B．between 30 and 45 minutes
C．between 15 and 30 minutes
D．one hour or more
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单项选择题
In Paragraph 5, the author refers to a hypothesis as a leap into the unknown in order to show that hypotheses ______.
A．go beyond available facts
B．are sometimes ill-conceived
C．can lead to dangerous results
D．require efforts to formulate
单项选择题
Science involves ______.
A．imagination and creative thinking
B．collecting information
C．performing experiments
D．all of the above
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