The correct order of density of $Be$,$Mg$,$Ca$,$Sr$ is

Which of the following alkaline earth metals does not exhibit colour when heated in a flame?

The correct order of density of $Be, Mg, Ca, Sr$ is

Match the compounds in List-$I$ with their chemical formulas in List-$II$ and select the correct answer using the codes given below:
List-$I$ (Compounds) | List-$II$ (Formulas)
$(A)$ Plaster of Paris | $(i) \, CaSO_4 \cdot 2H_2O$
$(B)$ Epsomite | $(ii) \, CaSO_4 \cdot \frac{1}{2} H_2O$
$(C)$ Kieserite | $(iii) \, MgSO_4 \cdot 7H_2O$
$(D)$ Gypsum | $(iv) \, MgSO_4 \cdot H_2O$

What is the purpose of adding gypsum $(CaSO_4 \cdot 2H_2O)$ to cement?

Plaster of Paris is hardened by