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B, the cell membrane

Explanation:

The cell membrane dictates what enters and exits the cell. It keeps the important stuff (such as organelles) inside the cell, allows nutrients to enter the cell, and keeps unwanted ¨things¨ from entering. The tea bag does roughly the same thing.

It keeps the important contents inside (in this case the tea leaves), allows water to come inside, and keeps whatever is in the water outside of the teabag. Obviously the cell membrane is much more complicated than a tea bag, but the tea bag works as a nice analogy.