若 $\frac{1+7i}{2-i}$ =a+bi（a，b∈R），i是虚数单位，则乘积ab的值是（   ）

如图是地球公转示意图（阴影部分表示黑夜）。当我们这里每年处于白天最长，黑夜最短的季节时，地球处于图A、B、C、D的什么位置（　　）

下列关于染色体的叙述中，不正确的是(　　)

函数 $f\left(x\right)=\text{sin}x$ ，对于 $x_1<x_2<x_3<\cdots <x_n$ 且 $x_1,x_2,\cdots ,x_n\in \begin{array}{cc}0,& 8\pi \end{array}$ （ $n\ge 10$ ），记 $M=\left|f\left(x_1\right)-f\left(x_2\right)\right|+\left|f\left(x_2\right)-f\left(x_3\right)\right|+\left|f\left(x_3\right)-f\left(x_4\right)\right|+\cdots +\left|f\left(x_{n-1}\right)-f\left(x_n\right)\right|$ ，则 $M$ 的最大值等于____．

      Computers can help many workers. They can also help thieves.
      There was one thief who worked in a bank of the United States. He paid for everything he needed by check (支票). Then he told the computer not to take any money from his account for the checks he wrote. The bank's computer did what he said. And it did not take any money from his bank account. One day the computer broke down. People had to do the computer's work. They soon found out what was happening.
      Another thief, who worked for a bank saw people filling in deposit forms (存款单). Many of these people did not know their account numbers, so they wrote only their names on the forms. They left empty places for their account numbers. The thief wrote his own account number in this place. When the computer read these forms it looked only at the account numbers. It did not look at the customers' names. It paid the money on these deposit forms into the thief's account .                           
       Then there was the bank worker who told the bank computer to take ten cents off every customer's account each month and put it into his own account. For a long time the bank's customers did not notice that they were losing ten cents every month. But then someone did notice and told the bank. That was the end of that little game!