被m除得的余数相同，则称a和b对模m同余，记为a=b（bmodm）．若 $a=C_{20}^0+C_{20}^1\cdot 2+C_{20}^2\cdot 2^2+\cdots +C_{20}^{20}\cdot 2^{20}$ ，a=b（bmod10），则b的值可以是（   ）

（+表示实施该步操作，﹣表示缺少该步操作）

同一纬线上各地正午太阳高度和昼夜长短相同。

    One day, several young people went for a picnic. They walked in the open air with 1.

    As soon as they came to a river, they 2 by the clear and calm water. They played with cool water excitedly. Suddenly, a lovely 3 passed by Tom. He tried to catch it, 4 the fish was just swimming a few feet ahead of him. He ran 5 it, hoping to catch it.

    Gradually, Tom came to the water with waist deep. He felt 6, so he wanted to jump out of the water. 7 up and down in the water, he went farther and farther away from the bank. Just at that time, Barry saw this, he offered 8 help to Tom at once. Tom saw someone 9 to him, he caught the person 10 was close to him. Unfortunately, Barry was dragged into the water. The two struggled in the water 11.  Amy, on the bank, noticed that, and she 12 others near the bank.

    Hearing their cries, two farmers near the river took a long rope there. They threw one end of the rope to 13 of them in the water. Together, all the people on the bank pulled 14 end.

    Finally, the two boys were saved!

    The experience has told us 15 we should do something to help others correctly.