aboutfour metres long, the room served as a small reception hall for importantvisitors .
问：这句中about four metres long，在这句中应该被称作主句的什么状语？
一些短语，包含名词性短语或分词短语可以作说明性状语，用来描述句子的背景状况。可以转换成一个并列句。
1. Founded in1900, the university has a history of more than 100 years. =
The university wasfounded in 1900, so it has a history of more than 100 years.
2. about fourmetres long, the room served as a small reception hall for important visitors.=
The room was aboutfour metres long, and served as a small reception hall for importantvisitors.
