多选题 People did not suddenly learn to use fire, but did so slowly over time with countless advances offset by ______ period.

【正确答案】 A、F

【答案解析】[解析] ●offset说明前后反义重复。
●空格与advances(进步)反义重复。
●recessive退步的，fertile肥沃的，accumulated增长的，contemplative沉思的，explosive爆炸的，regressive退步的。答案选AF。