Part B Directions: You will hear 3 conversations or talks and you must answer the questions by choosing A, B, C or D. You will hear the recording ONLY ONCE. 听力原文： As you all know, log structures are gaining popularity. They are no longer just the simple country homes which we think of as the traditional log cabin. Some upscale homes now incorporate natural round logs in sealing beams and walls. People seem to think that the rounded logs give their homes a cozy warm atmosphere. And even people who want to build a traditional log cabin on their own can buy a kit with precut logs that fit together like pieces of jigsaw puzzle. Before showing you some slides of modern log houses, I'd like to introduce a little historical background on the subject. Log cabins were first built in the late 1600s along the Delaware River Valley. The European immigrants who settled there brought centuries of old traditions of working with logs. And in this heavily wooded area logs were the material in hand. Log cabins were the most popular in the early 1800s with the settlers who were moving west. They provided the answer to the pioneer's need for a safe and sturdy home that an ordinary family could build quickly. They had dirt floors and sliding boards for windows. But the log buildings that have probably had most influence on modem architects are those of the mountain retreats of wealthy New Yorkers. These country houses which were popular in the early 1900s typify what's known as the Adoroundyx style. Now let's look at those slides. What is the speaker mainly discussing?