A、They have a history of 14 centuries. B、They are 40 metres tall on average. C、They are without foundations. D、They consist of s

Dating from as far back as the 12th century, they are claimed to be the rarest historic buildings in western Europe. These buildings offer vital insights into Scandinavia’s Viking past. But now, with only 30 wooden churches remaining and their condition deteriorating, experts are working to preserve the structures for future generations. Some of these spectacular churches are no more than small buildings, barely four metres wide and six metres tall. Others are much larger structures that soar up to 40 metres into the cold air. Most consist of timber frames that rest on stone blocks. This means that they have no foundations. Although many of the churches appear from the outside to be complex structures, they normally feature only a single storey but numerous different roof levels.
   Staff from the Norwegian government have carried out conservation work on 10 of the churches over the past two years. Most of these churches date from between the 12th and 14th centuries. Other churches were conserved in previous years. So far, specialists have worked to add preservative materials to the churches’ exteriors. They also replaced rotting roofs and halted the sinking of the churches into the ground. In two cases, huge machines have been used to lift the buildings up to 30 centimetres into the air. This was accomplished so that the team could examine and repair the churches’ original medieval stone blocks. The team plans to return to around a dozen of the buildings to assess progress and consider further action. The earliest free-standing wooden church was probably built in Norway in around 1080. However, the largest known wooden churches were built from the 1130s onwards. This period was one of inter-elite rivalry in which nobles sought to increase their influence by funding the construction of churches and other buildings.
   The reason for constructing the buildings from wood is probably that ideally proportioned straight and slender timber was available in large quantities in Scandinavia’s vast pine forests. As wood was so plentiful, it was cheaper to use than the stone used in the buildings of other European cultures. The area’s ship-building tradition, partly established by the Vikings, also meant that sophisticated carpentry was a major aspect of the local culture. The complex style of the medieval wooden church carvings, and the skills used to make them, almost certainly derive from the ancient Viking tradition.
Questions 16 to 18 are based on the recording you have just heard.
16. What does the speaker say about the Viking wooden churches?
17. What is special about most of the Viking wooden churches?
18. Why were the Viking churches constructed from wood?