问题：
[单选题]已知甲乙两个方案投资收益率的期望值分别为10％和12％，两个方案都存在投资风险，在比较甲乙两方案风险大小时应使用的指标是（ ）。
A、标准离差率
B、标准差
C、协方差
D、方差
参考答案：
A
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