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The experiment described in paragraph 5 caused Kramer to conclude that birds possess a biological clock because A. when birds navigate they are able to compensate for the changing position of the Sun in the sky B. birds innate bearings keep them oriented in a direction that is within 15 degrees of the Suns direction C. birds' migration is triggered by natural environmental cues, such as the position of the Sun D. birds shift their direction at a rate of 15 degrees per hour whether the Sun is visible or not