Chem Help! Limiting and excess reagent?

Identify the limiting and excess reagent for each of the following scenarios involving the formation of silicone dioxide, SiO2. Please explain!!! Thank you!

Si + O2 --> SiO2

a. 2 atoms of silicone and 2 molecules of oxygen gas
b. 28.1 g of silicone and 48.0 g of oxygen gas
c. 6 mol of silicone and 2 mol of oxygen gas