同时掷3枚硬币，那么互为对立事件的是( ) A．至少有1枚正面_爱下问搜题

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问题选择题

同时掷3枚硬币，那么互为对立事件的是(　　)

A．至少有1枚正面和最多有1枚正面

B．最多1枚正面和恰有2枚正面

C．至多1枚正面和至少有2枚正面

D．至少有2枚正面和恰有1枚正面

答案

答案：C

本题考查对立事件的概念.

同时掷3枚硬币，至少有1枚正面包括三种情形：“一正两反”、“两正一反”、“三正”，其对立事件为“三反”，故A错.

同时掷3枚硬币，最多1枚正面包括：“一正两反”、“三反”，其对立事件为“两正一反”和“三正”，故B错；

同时掷3枚硬币，最多1枚正面包括：“一正两反”、“三反”，其对立事件为“两正一反”和“三正”，即至少两枚正面故B错，而C正确.

同时掷3枚硬币，至少2枚正面包括：“一正两反”、“三正”，其对立事件为“两正一反”和“三反”，即至多 1枚正面，故C错；

正确答案C

填空题

You might think that hamburgers were invented in the United States, but that is not totally true. These (1) , or patties, actually came from Germany in the middle of the nineteenth century. They were brought to the United States by (2) who came from the city of Hamburg. That is why (3) was "hamburger steak".

However, people in other places (4) that they invented the hamburger. Perhaps we’ll never have a (5) . But there’s no question that the hamburger was a (6) . Why Perhaps because at that time, industry was growing and a kind of (7) food was needed for workers.

The hamburger became even more popular (8) when the first chain of fast food restaurants was started. This chain was called "White Castle". It served tiny hamburgers that were sold for only (9) . Then, in the 1930s there came the (10) where customers were served in their cars by waiters in uniform. And the hamburger was one of the most (11) .

By now, The hamburger was ready to (12) the world. And this happened with McDonald’s, which was actually a (13) at first. But by the early 1950s the hot dog was replaced by the hamburger. McDonald’s and other fast food restaurants (14) around the world throughout the rest of the twentieth century. McDonald’s alone has (15) for everyone person in the world.

The importance of the hamburger to (16) remains significant. About (17) of all sandwiches that are eaten are hamburgers. According to some sources, (18) of current workers in the United States had their first job at McDonald’s. But the face of the hamburger is changing (19) . Nowadays it is possible to buy a chicken burger, a turkey burger, (20) , or veggie burger.

18（）

问答题

苍术防风汤功效与作用