问题
选择题
同时抛两枚硬币，则一枚朝上一枚朝下的事件发生的概率是（ ）
A．1／2
B．1／3
C．1／4
D．2／3
答案
答案：A
题目分析：分两种情况：可能第一枚朝上第二枚朝下，也可能第一枚朝下第二枚朝上。朝上时概率为
，则朝下时概率为
。故所求概率为
。
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同时抛两枚硬币，则一枚朝上一枚朝下的事件发生的概率是（ ）
A．1／2
B．1／3
C．1／4
D．2／3
答案：A
题目分析：分两种情况：可能第一枚朝上第二枚朝下，也可能第一枚朝下第二枚朝上。朝上时概率为，则朝下时概率为。故所求概率为。
I was at the post office early that morning, hoping to be in and out in a short while．Yet, I 36 myself standing in a queue that went all the way into the hallway．I had never seen so many people there on a weekday．It seemed someone might have made an announcement, welcoming customers to carry as many 37 as they could and bring them in when I needed to have my own package 38 ．The queue moved very slowly．My patience ran out and I got 39 ．The longer it took, the angrier I became．When I got to the counter finally, I finished my 40 quickly and briefly, and then walked past the queue that was now extending past the front door． "Excuse me," I said, trying not to be too pushy．Several people had to move 41 to make room for me to get to the 42 ． I stepped out, complaining about the 43 conditions．Thinking I was going to be late for my dentist appointment, I headed into the parking lot． A woman was coming across the lot in my 44 ．She was walking with determination, and each step sounded very heavy．I 45 that she looked as if she could breathe fire．It stopped me in my tracks．I 46 _ myself and it wasn't pretty．Had I looked like that? Her body language said that she was having a _47 day．My anger melted away．I wished I could wrap her in a hug but I was a 48 ．So I did what I could in a minute 49 she hurried past me－I smiled．In a second everything changed．She was astonished, then somewhat 50 ．Then her face softened and her shoulders 51 ．I saw her take a deep breath．Her pace slowed and she smiled back at me as we passed each other． I continued to smile all the way to my _52 ．Wow, it's amazing what a simple smile can do． From then on, I became aware of people's 53 and my own, the way we show our feelings．Now I use that _54 every day to let it 55 me that when facing the world, I can try a smile．
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