Directions: Complete the following passage by using ONE word that fits the context.

Because neither she nor her husband smoked, Mrs. Trench was surprised to see cigarette ash on her doorstep as she entered the house. When she opened the living-room door, she was astonished to see a strange man fast      48     in an armchair! Taking care not to disturb him, Mrs. Trench left the house at once. She called a taxi and went      49     to the police station. When she got there, she lost no time to explain             _____50    had happened and added that the man      51     have got into the house      52    an open window. Mrs. Trench returned home       53      a police car together with two policemen. But it was too late: the man had disappeared. Hurrying upstairs, she went to her dressing-table. She smiled       54       relief when she saw the only thing that man had taken was an imitation diamond necklace that was almost      55      !

假设某公司每年需外购零件4900千克，该零件单价为100元，单位储存变动成本400元，一次订货成本5000元，单位缺货成本200元，企业目前建立的保险储备量是50千克。在交货期内的需要量及其概率如下：

按企业目前的保险储备标准，存货水平为多少时应补充订货