Stem cells can form many kinds of cells. in contrast, most body cells cannot form different types of cells. for example, skin cells can only make skin cells and nerve cells can only make nerve cells. which statement best explains why skin cells will never become nerve cells?

a.) each type of cell receives a different message from the central dna, which is stored in dna cells.

b.) each type of cell has only the part of the dna necessary for making that type of cell.

c.) each cell type is determined by messages sent from the brain, which directs development.

d.) both types of cells have the same dna, bt the cells have different genes turned on resulting in a different type of cell.

Our planet has experienced five major extinctions in the four billion year history of life. the first, 450 million years ago, occurred shortly after the evolution of the first land-based plants and 100 million years after the cambrian explosion. the second extinction occurred 350 million years ago, causing the formation of coal forests. next earth experienced two mass extinctions during the triassic period, between 250 and 200 million years ago. the fifth mass extinction occurred 65 million years ago, ending the reptilian dominance of the earth. according to richard leakey, the sixth mass extinction is happening right now. leakey suggests that we, the human race, are the cause. each year, at our hand, approximately 50,000 species vanish from earth. he believes that man is destroying earth at a rate comparable with the impact of a giant asteroid. leakey's statistics indicate that 50% of earth's species will become extinct within the next 100 years assuming leakey's hypothesis of a sixth mass extinction to be true, how will we expect the model to change? a) a sharp spike in the graph approximately 100 million years from now b) a dip in the graph, followed by a sharp spike about 100 million years from now c) a sharp spike in the graph immediately following the "0" location of the x axis d) a plateau following the "0" mark on the x axis, followed by a gradual rise to a new peak