I believe it is the cell membrane
A shield<span> is generally a large area of exposed Precambrian crystalline igneous and high-grade metamorphic rocks that </span>form<span> tectonically stable areas. In all cases, the age of these rocks is greater than 570 million years and sometimes dates back 2 to 3.5 billion years.</span>
A variable.
In an experiment, it is important that everything is exactly the same in order to achieve correct results. The only exception are the variables, the independent variable and the dependent variable.
