21.D细节理解题。根据Waik1一一The Natural World部分中的“Valk Duration:6 hours'”、Walk2一一Introduction to Hillwalking部分中的“Walk Duration:5 hours'”、Walk3一一一一Moonlight Under the Stars部分中的“Walk Duration:3 hours'和/alk4一一一一PhotographicWa业in Kilbrannish Forest部分中的“Walk Duration:1.5 hours'”可知,Photographic Walk in Kilbrannish Forest用时最短,为1.5小时。故选Do22.A细节理解题。根据Waik3一一一一Moonlight Under the Stars部分中的“A torch along with suitable clothing is essential for walkingin the dark.Those who are dressed unsuitably will not be allowed to take part."可知,Walk3的参与者必须穿合适的衣服。故选Ao23.D细节理解题。根据Walk1一一The Natural World部分中的“Date and Time:Saturday”、Walk2一一Introduction to Hillwalking部分中的“Date and Time:Sunday”、Walk3-一一一Moonlight Under the Stars部分中的Date and Time:Saturday"和Walk4-一一一PhotographicWal业k in Kilbrannish Forest部分中的“Date and Time:Sunday'”可知,这四条徒步路线的共同之处是都安排在周末。故选D。