Given below are two statements: One is labelled as Assertion A and the other is labelled as Reason R.

Assertion A : Sodium is about 30 times as abundant as potassium in the oceans.

Reason R : Potassium is bigger in size than sodium.

In the light of the above statements, choose the correct answer from the options given below

For a good quality cement, the ratio of lime to the total of the oxides of $$\mathrm{Si}, \mathrm{Al}$$ and $\mathrm{Fe}$ should be as close as to :

Given below are two statements:

Statement I : Lithium and Magnesium do not form superoxide

Statement II : The ionic radius of $$\mathrm{Li}^{+}$$ is larger than ionic radius of $$\mathrm{Mg}^{2+}$$

In the light of the above statements, choose the most appropriate answer from the options given below :

What is the purpose of adding gypsum to cement?