~~9. genetically speaking, why is it important not to mate with a close relative

While linebreeding is less likely to cause problems in the first generation than does inbreeding, over time, linebreeding can reduce the genetic diversity of a population and cause problems related to a too-small gene pool that may include an increased prevalence of genetic disorders and inbreeding depression

Explanation:

They can, do, and have done so!  

The reason it's bad to do it for several generations, is because there is a lack of genetic variety, and genetic diseases eventually begin to appear, such as hemophilia, deformities, etc.  

The British Royalty was an example of this, several hundred years ago!

Explanation:

d. planting trees

the trees take in carbon to make oxygen.