【正确答案】 1、a framework of heavy tree trunks    

【答案解析】这是对Norsemen建造房屋的方式的表述，故可定位于第四段。根据第四段第一句When the Norsemen came from Scandinavia to northern Europe， they found many forests，So they built homes with a framework of heavy tree trunks and they filled the space between the trunks with clay可得本题答案。