OG6 阅读 Q23

Choice A is the best answer. The passage says that to test their hypothesis, the scientists "planted 168 Texas gourd vines in an Iowa field" (lines 33-34,"To...and") and then ultimately walked "from flower to flower, observing each for two-minute intervals" (lines 62-63,"several...and writing"). Because they gathered data by looking at and studying the plants in question, the scientists' research is best characterized as relying on direct observation. 
Choices B, C, and D are incorrect because lines 62-63("several...and writing") make clear that the research emphasized direct observation, not historical data (choice B), expert testimony (choice C), or random sampling (choice D).