Answer:
a) reaction with oxygen
2mg +o2---------2mgo
b) Agno3+NaCl ----------AgCl+NaNo3
Its b because it explains it better than a waterfall does
CH4 + 2O2 =======> CO2 + 2H2O
Answer : The limiting reagent is 
Solution : Given,
Moles of methane = 2.8 moles
Moles of
= 5 moles
Now we have to calculate the limiting and excess reagent.
The balanced chemical reaction is,

From the balanced reaction we conclude that
As, 2 mole of
react with 1 mole of 
So, 5 moles of
react with
moles of 
From this we conclude that,
is an excess reagent because the given moles are greater than the required moles and
is a limiting reagent and it limits the formation of product.
Hence, the limiting reagent is 
I would say it would be a. It makes the most sense