Answer:
A) Iris
Explanation:
The Iris controls the diameter of the opening in your eye, thus also controlling the amount of light let inside. It's also reponsible for the colour of you eye.
Because if you change two things then you do not know which one has affected or altered the dependent variable. if you only change one then you know what exactly changed and why
Answer:
The mass ratio of oxygen rounded to the nearest whole no. 3 : 2
Explanation:
According to Law of Multiple proportion when two elements combine to make two or more different compounds, the mass ratio of the two element in the first compound, when divided by the mass ratio of the second compound , form a simple whole number ratio.
Compound A contains 1.34 g of sulfur for every 0.86 g of oxygen

Compound B contains 11.63 g of sulfur for every 10.49 g of oxygen

Ratio of oxygen in each compound
always put the larger number over the smaller number.

In order to form new molecules, a chemical reaction would have to occur which means the change would be a chemical change.