单选题

During the early period of ocean navigation,（）any need for sophisticatedinstruments and techniques.

Aso that hardly

Bwhen there hardly was

Chardly was

Dthere was hardly

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正确答案：A （备注：此答案有误）

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单选题

During the early period of ocean navigation,（）any need for sophisticatedinstruments and techniques.

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ecting a beam of light to a mirror located kilometers away and placed a rotating cogwheel（旋转齿轮）between the beam and the mirror. From the rate of rotation of the wheel,number of wheels teeth and distance of the mirror,he was able to calculate that the speed of light is 313 million meters per second. In a vacuum（真空）,however,the speed of light is 299,792,458 meters per second. This is about a million times faster than the speed of an airplane.Objects are visible to the human eye as light is______.

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directing a beam of light to a mirror located kilometers away and placed a rotating cogwheel（旋转齿轮）between the beam and the mirror. From the rate of rotation of the wheel,number of wheels teeth and distance of the mirror,he was able to calculate that the speed of light is 313 million meters per second. In a vacuum（真空）,however,the speed of light is 299,792,458 meters per second. This is about a million times faster than the speed of an airplane.Stars,flames,flashlights are some examples of______.

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