Which of the following substances is NOT in protein?

1. A

   Cotton

   (a) Cotton. Cotton fibre is almost pure cellulose, not protein. That is the official ‘NOT a protein’ key.

2. B

   Hair

   (b) Hair. Keratin, a fibrous protein.

3. C

   Spider silk

   (c) Spider silk. Fibroin protein.

4. D

   Hoof

   (d) Hoof. Keratin protein, like horn and nail.

Summary. Official key is (a) Cotton. Cotton is cellulose; hair, spider silk and hoof are proteins (keratin/fibroin). Honour the stored letter (a).