Four substances, labeled W, X, Y, and Z, are shown below.

Identify the letters of the two substances from the diagram that can be formed by burning a single element in oxygen.
Practise Edexcel GCSE Chemistry Fuels with exam-style questions for Foundation and Higher tier. 100 questions, matched to the Edexcel GCSE Chemistry (1CH0) specification and written in Paper 1 and Paper 2 style. Every question includes a full worked solution and mark scheme, so you can see where marks are awarded rather than just whether you got the answer right.