Select the correct statement(s):
a. When Al is added to potassium hydroxide solution, hydrogen gas is evolved
b. H2SiF6 is formed when silica reacts with hydrogen fluoride followed by hydrolysis
c. Phosphine gas is formed when red phosphorus is heated with NaOH
d. (NH4)2SO4⋅FeSO4⋅6H2O is called alums