The cyclic compound shown is heated with a small amount of water to bring about polymerisation. Select all correct statements.

Caprolactam opens by ring-opening polymerisation to give nylon 6, a polyamide, and no small molecule is eliminated in the ring-opening propagation steps. The repeat unit is -NH-(CH2)5-CO-, with five CH2 groups (the ring has six carbons, one being the carbonyl carbon), and the links are amides, not esters. So statements A and B are correct, and C and D are wrong.