Cells grown in a petri dish tend to divide until they form a thin layer covering the bottom of the dish. if cells are removed from the middle of the dish, the cells bordering the open space will begin dividing until they have filled the empty space. what does this experiment show?

a when cells come into contact with other cells, they stop growing.
b the controls on cell growth and division can be turned on and off.
c cell division can be regulated by factors outside the cell.
d all of the above

Option B, The controls on cell growth and divisions can be turned on and off

Explanation:

There are signal proteins that control and regulate the cell growth and division process. For instance, mitogens help in removing the negative controls which would otherwise have blocked the cell cycle progression. Then there are growth factors that synthesize proteins and hence stimulate cell growth. Also there are some survival factors that suppress the apoptosis. Based on these three factors the cell division and growth can be turned on or off.  

Hence, option B is correct

the atmospheric gas that forms a mild acid when dissolved in water is called carbon dioxide. the correct answer is a. 2.) chemical weathering would cause an inscription on a marble headstone to become harder and harder to read over time.