单选题 Probably every town in the United States has a post office. Some are very small, and you may find them in the corner of a shop. Others are large buildings. They are open five days a week and on Saturday mornings. From Monday through Friday they are usually open from 8:30 to 4:30.
If you know how much the postage (邮资) is for your letter, you can buy stamps at any window. In some post offices you can buy stamps from machines. Stamps are sold at many different prices, from one cent to many dollars. If you are not sure how much postage is for your letter, you can ask the clerk. He or she will give you the stamps you need.
If you are sending your letter far away, write on the envelope that you want them to go by air or use an airmail envelope. Remember that postage will be more expensive for a letter that you want to send overseas (海外).
At a post office you can also buy postcards. A postcard is cheaper than a letter. Usually the postage for a postcard is about half that of a letter. The postcards that you buy at a post office do not have pictures, however. Also they are not to be sent outside the country.