重写句子：Alan passed the test. Alan had studied hard. Alan had understood the material.(complex sentence)
Alan passed the test because Alan had studied hard and understood the material.
暂无解析
机电设备的任何故障，都必须及时排除，确保其处于完好状态
慢性阻塞性肺疾病治疗的目的有（）
简述我国人口老龄化的主要特点。
当羧酸中的a一碳上引入吸电子基团时,其酸性要比乙酸强。
实际应用时,指示剂的变色范围越窄越好。