Question: Given the following passage  "During this period, the island enjoyed increased revenues through the sale of flax, with prices peaking in 1951. However, the industry declined because of transportation costs and competition from synthetic fibres. The decision by the British Post Office to use synthetic fibres for its mailbags was a further blow, contributing to the closure of the island's flax mills in 1965.",  answer the following question. Note that the answer is present within the text.  Question: It is implied that the British Post Office used to make their mailbags out of what fibres?
Answer:
flax