问题
单项选择题
在工程网络计划中，M工作的最早开始时间为第16天，其持续时间为4天，该工作有三项紧后工作，它们的最早开始时间分别为第22天、第23天、第25天，则M工作的自由时差为( )天。
A．4
B．2
C．3
D．5
答案
参考答案：B
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在工程网络计划中，M工作的最早开始时间为第16天，其持续时间为4天，该工作有三项紧后工作，它们的最早开始时间分别为第22天、第23天、第25天，则M工作的自由时差为( )天。
A．4
B．2
C．3
D．5
参考答案：B
When my twin sons, Chad and Brad, were born, I was concerned about everything. Five years later, our little girl, Becky 1 our family. I wanted everybody to be healthy and happy. I worked hard to see that they were. As the kids grew older, I worried about headaches, throat infections and many other common childhood illnesses．I didn’t like it when the boys spent time “warming the bench (板凳)” during Little League football games. I worried about Becky when she 2 the ball while playing softball．Before long, the teen years were upon us. I stayed up late at night waiting for the boys to return home．Many times the 3 crossed my mind that I would call the police if they weren’t home on time. 4 , they always arrived home safe and sound before I had to take such 5 . “Please don't ever call the police,” one of the boys said when I 6 him after a late arrival. The day the boys moved away to college was a(n) 7 day indeed. I worried about their being able to take care of themselves. Would they starve? A few months after the boys left college, our 8 rang in the middle of the night. It startled (惊醒) us when we looked at the clock．It was 3 o’clock in the morning. “Something must be wrong,” I shouted to my husband, Roy, as we both jumped up．We ran to the door, opened it, and there stood a police officer. “You need to call your sons,” he 9 announced. I picked up the telephone, but unfortunately, it was 10 . A line outside had been accidentally cut. Roy and I jumped into the car and 11 to the nearest telephone．My stomach ached．My husband was 12 so badly that he could hardly dial the number. On the first 13 , Chad answered the telephone. “What's wrong?” Roy shouted into the receiver (听筒). “We were worried about you,” Chad told him. “We’ve been trying to call you all night, but you didn’t 14 .We called the police and asked them to go and check on you.” For the first time in their lives, the boys were worried about us. And they were the ones who 15 had to call the police.
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