附:(随机变量ξ服从正态分布N(μ,δ2),则P(μ﹣δ<ξ<μ+δ)=68.26%,P(μ﹣2δ<ξ<μ+2δ)=95.44%
A:6038
B:6587
C:7028
D:7539
龙羊峡大型水电站所在的省区和水系是( )
I____(thought/ think) it was easy to do before.