What is carbon fibre made of?

Quite obviously, carbon fibre is made from mostly carbon. However, I have to explain why it is a composite material by explaining that it is made up of more than one element. Luckily, I’m only required to say A and B are included. I’ll say carbon as A, but what could I say for B?

Thanks.
I forgot to mention — it’s because I’m doing a GCSE Engineering paper in a few days time and I believe this is a common question they like to ask.

If you think about it, what other than carbon, is seen to have the same structure. Carbon fibre is also known as Graphite fibre, very thing fibres. So B could be Graphite.