Which of these are pure substances, assuming there is no phase change?

Question

Consider the following:        

I . Air

2. Gaseous combustion products

3. Steam

Which of these are pure substances, assuming there is no phase change?

(a) 1 and 2 only           (b) and 3 only

(c) 2 and 3 only           (d) 1, 2 and 3