关于正态曲线性质的叙述:①曲线关于直线x=μ对称,整条曲线在x轴上方;②曲线对应的正态总体概率密度函数是偶函数; ③曲线在x=μ处处于最高点,由这一点向左右两边延伸时,曲线逐渐降低; ④曲线的对称位置由μ确定,曲线的形状由σ确定,σ越大曲线越“矮胖”,反之,曲线越“瘦高”.上述对正态曲线的叙述正确的是( )A.①②③ B.①③④ C.②③④ D.①②③④The library is full of students ____ for the coming exam. A. busily prepared B. busy preparing C. busily prepare D. are busy preparing