Answer:
The correct answer is None of these choices are correct.
Explanation:
Crossing over doesn't occur only between heterozygous genes, it also doesn't happen only in some chromosomes and it never reduces de variation, otherwise, the crossing over is one of the main causes of variation among species.
There was no life on Earth for the first billion years because the atmosphere was not suitable for life. Earth's first atmosphere had lots of water vapor but had almost no oxygen. Later, frequent volcanic eruptions put several different gases into the air.
<span>Choice (a) is the most correct. Fish must excrete large quantities of water as a way of keeping the homeostasis in their cells. Otherwise, the water levels in the cell could become too high and the cells would likely rupture, leading to the death of the organism.</span>
