Which one of the following substances is NOT synthetic?

1. A

   Fibroin

   Fibroin is the structural protein of natural silk from Bombyx mori (and other silkworms). It is not a laboratory polymer, so it is the non-synthetic substance and the official key.

2. B

   Lexan

   Lexan is a brand of polycarbonate, a synthetic engineering plastic.

3. C

   Neoprene

   Neoprene is polychloroprene, a synthetic rubber developed as an oil-resistant elastomer.

4. D

   Teflon

   Teflon is polytetrafluoroethylene (PTFE), a fully synthetic fluoropolymer.

Summary. Official key is (a) fibroin. Silk fibroin is a natural protein fibre; Lexan, neoprene and Teflon are industrial synthetics. Do not treat ‘fibre’ as automatically synthetic. Honour the stored letter (a).