ls说的好像有点绝对,which也可以指代带介词短语的中心词
比如:
Emily Dickinson's letters to Susan Huntington Dickinson, which were written over a period beginning a few years before Susan's marriage to Emily's brother and ending shortly before Emily's death in 1886,outnumber her letters to anyone else.Unlike most other mergers in the utility industry, which have been driven by the need to save money and extend companies’ service areas, the merger of the nation’s leading gas company and leading electric company is intended to create a huge network for marketing the utilities in question as states open their utility markets to competition.-- by 会员 crazyxxx (2010/6/11 16:42:20)