There are 3 reading passages in this part. Each passage is followed by some questions or unfinished statements. For each of them there are four choices marked A, B, C and D. You should decide on the best choice and mark the corresponding letter on the Answer Sheet with a single line through the center.What does the passage mainly discuss A．The development of trade routes through United States citiesB．Contrasts in settlement patterns in United StatesC．Historical differences among three large United States citiesD．The importance of geographical situation in the growth of United States cities