It is important because the results would not be reliable since other variables you are not testing could have been in affect.
Hope this helps(;
The question doesn't have choices. It would be better to have one.
B. mitochondria and nucleus
<span>Evidence that human activities influence the global climate system continues to accumulate. Data indicate that Earth's surface temperature is rising. This increase can be attributed, in part, to human-caused increases in greenhouse gases such as carbon dioxide. It is becoming apparent that these climatic changes are negatively affecting physical and biological systems worldwide.</span>
