H2O is the missing reactant.
Just a caveat: this equation isn’t balanced.
Answer:
There are often not more than one or two independent variables tested in an experiment.
Answer : The correct option is, (A) silver ion gains electrons more easily and is a stronger oxidizing agent than a chromium(III) ion.
Explanation :
The given half reaction are :
1st half reaction : 
The reduction potential of this reaction = +0.80 V
2nd half reaction : 
The reduction potential of this reaction = -0.74 V
From the reduction potentials, we conclude that the reaction which have positive reduction potential, they will gain electrons more easily and reduced itself and act as a stronger oxidizing agent.
Or we can say that the reaction which have negative reduction potential, they will lose electrons more easily and oxidized itself and act as a stronger reducing agent.
The reduction potential of 1st half reaction is positive and 2nd half reaction is negative.
Therefore, the silver ion gains electrons more easily and is a stronger oxidizing agent than a chromium(III) ion.
because Magnesium oxide has a high melting point (2852°C) and so resists the high temperatures in a furnace.It is known as a refractory material - which just means that it is resistant to heat.
A water molecule, because of its shape, is a polar molecule.