19. Suppose that some cells are grown in culture in the presence of radioactive nucleotides for many generations, so that both strands of every DNA molecule are radioactive. The cells are then harvested and placed in new medium with nucleotides that are not radioactive, so that newly synthesized DNA will not be radioactive. What proportion of DNA molecules will be radioactive after two rounds of replication

1. emigration the movement of people from one place to another 2. immigration a situation in which resources are being used up at a faster rate than they can be replenished 3. migration the leaving of one's homeland to settle in a new place 4. overpopulation the movement of people to a new country 5. sustainable development a situation in which the birth rate is not sufficient to replace the existing population 6. underpopulation the ability to meet current needs without reducing the ability to meet future needs 7. unsustainable development the increase in the population of a city 8. urbanization a situation in which the number of people exceeds the available natural resources in an area

Which of the follow describes a disadvantage secondary sources have compared to primary sources?

The human body needs energy in order to carry out life processes such as breathing. where does the body get this energy? a. from eating food b. from learning about new things c. from lying in the sun d. from sleeping