What is the mixture of $MgCl_2$ and $MgO$ called?

The correct statements among the following are:
$(A)$ $BeH_2$ and $MgH_2$ are polymeric in nature.
$(B)$ $LiH$ is unreactive to oxygen at moderate temperatures.
$(C)$ $BeH_2$ and $MgH_2$ possess significant covalent character.
$(D)$ The stability of alkali metal hydrides follows the order $LiH < NaH < KH < RbH < CsH$.

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

Which of the following is the main component of Portland cement (besides lime)?

The initial setting of cement is primarily due to which of the following?

Which of the following reactions is incorrect regarding the hydrolysis of metal carbides?