21. Which among the following is incorrect about the layers of the cell wall?

a) The cell wall is made of three main layers, namely, primary cell wall, secondary cell wall and middle lamella

b) Cell wall of a young plant is made of primary cell which composes a loose network of cellulose microfibrils

c) Secondary cell wall forms as the cell matures and composes cellulose and lignin

d) Middle lamella separates cells and is composed of sodium acetate

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