It is interesting to visit another country, but there are some problems when we don't know the 1very well.It may be 2 to talk with the people there. We may not know how to use the telephone in the country that we are visiting. We may not know how to buy 3 we need. In a4country we might not know where to eat or what to order in a 5 It is not easy to decide how6to tip (给……小费) waiters or taxi drivers. When we need help, we might not know how to ask for help.It is not pleasant to have an experience7that. 8a short time, however, we learn what to do and what to 9We learn to enjoy life in another country and then we may feel 10 because we have to leave.

化简$\frac{a}{a-b}-\frac{b}{a-b}$的结果是（  ）

①氨基酸 ②C、H、O、N等元素 ③氨基脱水缩合 ④一条或几条多肽链连接在一起 ⑤多肽 ⑥蛋白质．

西夏学习汉族先进文化的措施不包括（     ）

显微镜的物镜成像相当于下列哪种光学元件的成像（　　）