They （）the evidence in the past. ["held","have held","are holding","have been holding"]
受助者
美国芝加哥大学社会学教授E．w．伯吉斯（Burgess）通过研究美国城市芝加哥，于1925年提出了城市地域结构的。（） ["扇形模式","带形模式","同心圆模式","多核心模式"]
I’ve lived here for years since I （）here . ["move","have moved","moved","would move"]
嫁接成活的原理
债券
