[听力原文] 女：这次比赛你赢了吗？ 男：哎，差一点儿就赢了。 问：关于男的，

施工单位以下做法符合《建设工程质量管理条例》规定的有()。

A.施工中发现图纸错误，经监理同意后按照强制性标准施工 

B.混凝土浇筑施工前，应当通知质量监督站见证取样 

C.施工人员与专业监理工程师去钢材加工厂对钢构件进行抽样检测 

D.拒绝接收不合格的甲供材料进场

Under one of the following conditions, goods or services may be procured through competitive negotiation in accordance with this Law: ______.

A．where, qualified tender is lacking，or re-invitation fails
B．where the goods or services in question are special in character and can only be procured from a limited number of suppliers
C．where goods or services can be procured from only one supplier
D．where consistency of the items or compatibility of the services procured requires procurement of additional items or services from the same supplier

设总体X的概率密度函数为

，其中λ＞0。X₁，X₂，…，X_n是总体X的一个容量为n的样本。

求参数λ的矩估计量；

怎样显示或隐藏站场操控大按钮？