1. _______________ （随着工商业的发展）,the number of trees in many big cities has dramatically reduced.
　　2. Travel can widen our knowledge, _____________ （扩大我们的眼界）,and make one open-minded as well.
　　3. Cars are responsible for most of the smog in cities, _______________ （这已经对环境造成了严重的污染）.
　　4.Modernization will be entirely possible in the first half of the 21st century, as long as we ________________（坚持认为）that economic development is the center of all our work.
　　5. He was convinced on the basis of ____________（当时已出现的情况）that there would be a long and difficult struggle.

　　答案与解析：
　　1. With industries and business developing quickly
　　2. broaden our horizon
　　3. which has seriously polluted the environment
　　4. hold an idea/ insist on the idea/ stick to the idea
　　5. what had appeared by then