我国幅员辽阔，各地方言不一致，汉文字却是统一的。这是因为（　　）

阅读理解
    A beautiful and very successful actress was the star for a new musical show. Her home was in the country, but she didn't want to go back there every night, so she bought an expensive house in the center of the city, got some beautiful furniture and got a man to paint the rooms in new colour.
    It was very difficult to get tickets for her show in the evening, because everyone wanted to see it. So she decided to give the painter two of the best seats. She hoped that this would make him work better for her. He took the tickets without saying anything, and she heard no more about them until the end of month, when she got the painter's bill. At the bottom of it were these words: “Four hours from 5:00 p.m. to 9:00 p.m. watching Miss Hall sing and dance: 3 pounds,”with this note: “Before 5:00 p.m. I get 10 shillings (先令)an hour and after 5:00 p.m. 15 shillings.”

日本资源匮乏，下列进口原料当中所占比重最低的是（ ）

如图所示，简谐横波a沿x轴正方向传播，简谐横波b沿x轴负方向传播，波速都是10m/s，振动方向都平行于y轴，t=0时刻，这两列波的波形如图所示．下图是画出平衡位置在x=2m处的质点，从t=0开始在一个周期内的振动图象，其中正确的是（   ）