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【单选题】
人类活动导致大气中含氮化合物浓度增加，产生沉降，是新出现的令人担忧的全球变化问题。一科研小组选择受人类干扰较小的某地．实验模拟大气氮沉降初期对植被的影响。实验地植被以灌木植物为主．伴生多年生草本植物。下表数据为实验地以2009年为基数，2010--2013年实验中植被的变化值(测量时间为每年9月30日)。据此完成21-23题。 年份 2009 2010 2011 2012 2013 灌木植物 1 1．oo 1．OO 1.00 1.00 植株数量 草本植物 1 1.18 1.20 1121 1.23 灌木植物 1 1.09 1.10 1.12 1.11 地上生物量 草本植物 1 1.47 1.55 1.52 1.53 灌木植物 1 1．O1 0.99 0.98 0.97 地下生物量 草本植物 1 1.21 1.29 1.42 1.58 实验期间植被变化表现为（ ）。 ①生物量提高②生物量降低③植株密度改变④植被分布改变 查看材料
A.
①③
B.
②③
C.
①④
D.
②④
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