搜题请搜索小程序“爱下问搜题”
问题 选择题

下列有关“DNA粗提取”的操作中，错误的是[ ]

A．在鸡血细胞液中加入蒸馏水，使其破裂

B．在“析出DNA黏稠物”时，要缓缓加酒精，直至溶液中黏稠物不再增多

C．在用酒精凝集的DNA时，要使用冷酒精

D．用玻璃棒搅拌时，要沿一个方向

答案

答案：B

填空题

景观中的生态流过程主要表现为（）和（）两种方式。
查看答案
单项选择题

Throughout the 19th century and into the 20th, citizens of the United States maintained a bias against big cities. Most lived on farms and in small towns and believed cities to be centres of (1) , crime, poverty and moral (2) Their distrust was caused, (3) , by a national ideology that (4) farming the greatest occupation and rural living (5) to urban living. This attitude (6) even as the number of urban dwellers increased and cities became an essential (7) of the national landscape. Gradually, economic reality overcame ideology. Thousands (8) the precarious (不稳定的) life on the farm for more secure and better paying jobs in the city. But when these people (9) from the countryside, they carried their fears and suspicions with them. These new urbanities, already convinced that cities were (10) with great problems, eagerly (11) the progressive reforms that promised to bring order out of the (12) of the city.

One of many reforms came (13) the area of public utilities. Water and sewerage systems were usually operated by (14) governments, but the gas and electric networks were privately owned. Reformers feared that the privately owned utility companies would (15) exorbitant (过渡的) rates for these essential services and (16) them only to people who could afford them. Some city and state governments responded by (17) the utility companies, but a number of cities began to supply these services themselves. (18) of these reforms argued that public ownership and regulation would (19) widespread access to these utilities and guarantee a (20) price.

（13）（）

A.at

B.by

C.out

D.in
查看答案