Which compound produces a paramagnetic acidic gas upon heating?

Reaction of $BeO$ with ammonia and hydrogen fluoride gives '$A$' which on thermal decomposition gives $BeF_2$ and $NH_4F$. What is '$A$'?

The most stable carbonate is

An alkaline earth metal $M$ readily forms water-soluble sulphate and water-insoluble hydroxide. Its oxide $MO$ is very stable to heat and does not have rock-salt structure. $M$ is :-

What is the use of Plaster of Paris?

Choose the correct statement$(s)$:
$A.$ Beryllium oxide is purely acidic in nature.
$B.$ Beryllium carbonate is kept in the atmosphere of $CO_2$.
$C.$ Beryllium sulphate is readily soluble in water.
$D.$ Beryllium shows anomalous behavior.
Choose the correct answer from the options given below: