According to the National Heart, Lung, and Blood Institute, adults need around seven to eight hours. For teenagers they recommend nine to ten, and at least ten for children.
<u>Answer</u>: c. a and b
Trees play vital roles in the production of oxygen as well as the fixation of carbon from 
. As trees use nitrogen, phosphorus and water as nutrients for growth, their diminishing numbers would change the balance between these three elements significantly.
Thus, during rains, the water level will rise much faster and higher in the absence of trees. These could possibly lead to increasing cases of devastating landslides.
<span>Great minds combined with technological advancements have a lead to creations and achievements that would have been impossible. Whatever great scientist of the past has achieved have already been surpassed or improved through modern and innovative ideas and processes. An excellent example of which is the direct manipulation of an organism’s DNA, also known as genetic engineering or genetic modification (GM). Coming from the terms genes and modify, the entire process is something out of a science fiction movie. Well, it used to be that way until the practice was perfected and used in actual applications. Genetic engineering made it possible to add new DNA or alter bad ones.</span>
TB. It is open to change.he thing about science is that a discovery may be made today and written up but by tomorrow something else is found that edifies what was found or proves it incorrect or make it obsolete. .Therefore the answer is <span>B. It is open to change</span>
Crossing over? That involves the random exchange of chromosomes.
