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The skin
Nonpolar lipid layers
Install air curtain
epiglottis
your boogers
Monopoly and Oligopoly are two barriers that prevent firms from entering the marketplace.
Shutting it.
Epiglottis
no
To prevent dangerous waste /sewer fumes from entering a structure
Phospholipids in the cell membrane prevent water from entering a cell.
Phospholipids in the cell membrane prevent water from entering a cell.
to close windows and doors
It is to prevent dust from wood from entering your eye.
Phospholipids
To prevent food entering the trachea
To prevent food entering the trachea