about, around, round; for which, of which, with whichHe showed t

about, around, round; for which, of which, with which
He showed the customers ______the plant, the construction ______had taken more than five years.
【正确答案】：around /of which
【题目解析】：介词在从句句首时，必须用关系代词which/whom，在此处用of which表示所有关系，补充完整句子即the construction of the plant。