The tallest building in the United States, the One World Trade Center, opened on November 3, 2014 in New York. The building is 1,776 feet tall, making it the 3rd tallest building in the world. It has 104 floors. The building stands in the area where the twin towers of the World Trade Center once stood. On September 11, 2001, a group of terrorists destroyed these twin towers and killed nearly 3,000 people. It took 8 years to build the new One World Trade Center. Does the height of the building in feet reminds you of something in history? The number is the year when the United States declared(宣布) its independence (独立)from Great Britain.

 One problem with such a tall building is how to clean the windows. Well, a platform(平台) hangs from ropes, and workers on the platform clean windows. On November 12, while two men were busy cleaning windows, a rope of the platform broke near the 68th floor. The two men were saved by cutting a hole in the glass window.

 The two tallest buildings in the world are the Burj Khalifa in Dubai, UAE and the Makkah Royal Clock Tower in Mecca, Saudi Arabia.

某校为了了解学生在校午餐所需的时间，抽量了20名同学在校午餐所需的时间，获得如下的数据（单位：分）：10，12，15，10，16，18，19，18，20，28，22，25，20，18，18，20，15，16，21，16．

若将这些数据以4分为组距进行分组，则组数是（　　）

《二十世纪中国的崛起》指出：“一个民族的觉醒，通常要有两个条件：第一，这个民族面对着或者经历了以往没有遇到过的严重危机，甚至整个民族被逼到生死存亡的关头，旧格局再也无法继续保持下去；第二，还要这个民族能看到出路，燃烧起新的希望，深信只要奋起救亡，勇于变革，就可以改变目前的艰难处境。”下列能够反映20世纪中国“旧格局再也无法继续保持下去”和“奋起救亡，勇于变革”的史实是（　　）​

对于非零自然数a和b，规定符⊗的含义是：a⊗b=$\frac{m\times a+b}{2\times a\times b}$（m是一个确定的整数），如果1⊗4=2⊗3，那么3⊗4=____ ．