It is difficult to discern the sample that is on the slide unless the microscope is adjusted properly.
【正确答案】 D
【答案解析】解析：句意：除非显微镜调整得适当，否则要辨别一个滑动的标本是很困难的。discern辨别，识别。detect察觉，探测。overlook忽略。disclaim放弃，不索求。discard丢弃。