30. all cells are surrounded by a semipermeable membrane, which regulates the passage of materials in and out of the cell. the principle structure of this membrane is
a. two layers of protein with a single layer of lipids in between.
b. a thick wall of cellulose with a thin lipid coating on the inside.
c. two layers of lipids with proteins associated with the lipids.
d. two layers of lipids with a polysaccharide layer in between.

carbohydrates: (starches and sugars) composed of carbon, hydrogen, and oxygen.

lipids: (fats, oils, and waxes) are made of carbon and hydrogen atoms.

nucleic acids: contain hydrogen, oxygen, nitrogen, carbon, and phosphorus. they are polymers of nucleotides. a nucleotide has three parts, a 5-carbon sugar, a phosphate     (-po⬇️4) group, and a nitrogenous base.

proteins: made up of nitrogen, carbon, hydrogen, and oxygen. they are polymers of amino acids.

common management practices in dairy and poultry farming are: (i) proper shelter facilities and their regular cleaning. (ii) some basic hygienic conditions such as clean water, nutritious food, etc. (iii) animals are kept in spacious, airy, and ventilated place.