Killing stoichiometry question

You burn 6 grams of carbon in plenty of air: C+O2=CO2 A. What volume of gas will form? (at rtp) B. What volume of oxygen will be used up?

Answers are 12 for both A and B

If you burn carbon in limited air the reaction is is different: 2C+O2=2CO a. What volume of gas will form this time? b. What volume of oxygen will be used up? Answers are a. 12dm3 b. 6dm3

Can someone explain me this 😷😷