在一个不透明的袋子中装有4个红球和2个白球，这些球除了颜色外无其他差别，从袋子中随机摸出一个球，则摸出白球的概率是____．

设为等差数列的前项和，且，，则（   ）

根据蔡增基《中国航运》记载：“自国民政府成立（1927年）至今，不过十年，此十年间，民营公司之组织，约计二十五家，……已远胜以前数十年矣。”当时航运业民营公司发展较快的主要原因是(    )

下列关于实验现象的描述中，不正确的是（　）

阅读下面短文, 从短文后各题所给的四个选项(A、B、C、D)中, 选出可以填入空白处的最佳选项。
     “Paul must have been trying to carry his waste paper to garbage can and dropped a few pieces.” I1picking them up. 2later I found more pieces. No quiet sighing this time. I 3 ,“Who is throwing garbage?” No answer. Instead, I saw more bits of paper silently floating 4 from upstairs. Looking up, I saw my seven-year-old son, Paul.
     “Stop making a mess.” “It's not a mess. They're5.” “Sorry, what did you say?” I hadn't heard him clearly.
    He didn't answer me. Paul has autism (自闭症) and6answers a question, especially when he's7attentively on something else. He ran down the stairs. “Where are my other butterflies?” he asked,8around. Every time Paul 9five or more words together, my heart says a 10of thanks. But lately he seems to 11that the benefits of forming complete sentences when communicating are12of the effort.
    Butterflies. Of course. I rushed to13them from the garbage,14them off and handed them to my young artist. “Want to see them15again?” he asked with a shy smile. “Oh yes! They're beautiful.” I whispered. He ran back upstairs to float his16 down again. They really did look like beautiful butterflies.
    That day Paul 17me to look up at 18instead of down at garbage. How many other masterpieces (杰作) do I miss because I'm too caught up in my 19to take time to appreciate what's right in front of me? Life is not what happens to us. It's 20 we look at it. Now, I look up.