Direction: Read the following paragraph and fill in the blank with the number. Paragraph Four: 1. Travel for business is an activity that can have a variety of different purposes, from scouting new business opportunities to entertaining clients. 2. One reason of business trips is to attend trade shows and conferences, since insights into new technologies and concepts are often provided on these occasions, and having employees there to represent the company may be useful to a business in a variety of ways. The topic sentence of the paragraph is_________ (写上面段落中的序号即可)