A.
Elements in the same group have similar properties.
B.
The similarity in their properties arises from the fact that they have an equal number of valence shell electrons.
C.
Fluorine, Chlorine, Bromine
The answer is A. Solids only
Are you sure it isn’t SO3+H2O = H2SO4 because that would be combination (synthesis) A+ B=AB
Or SO3 + H2SO4 = H2S2O7
Because that would also be synthesis
B is remaining the constant speed for 4 mins