In chemistry lab, a student took HCl in a beaker and was placed on a white paper with cross mark. When seen through the beaker the mark as shown in the figure was visible. On adding some sodium thiosulphate in the beaker the cross marked disappeared. When seen through the beaker. The student observed some reaction as the products formed were different than the reactants. Both the reactants were transparent.

1.Which of the following reaction is of similar
type as observed in the given set up.
(a) CaO(s) + H₂O(l) --> Ca(OH)₂(aq)
(b) 2FeSO₄ (s) --> Fe₂O₃(s) + SO₂(g) +SO₃(g)
(c) Fe(s) + CuSO₄ (aq) --> FeSO₄ (aq) +Cu(s)
(d) Na2SO4 (aq) + BaCl2(aq) --> BaSO₄ (s) +2NaCl (aq)

2.Name the resultant insoluble substance
formed in the flask.
(a) Sulphur
(b) Sodium sulphide
(c) Sodium sulphate
(d) Sodium carbonate

3.Which of the following reaction is done in
the above arrangement?
(a) Double displacement reaction
(b) Displacement reaction
(c) Precipitation reaction
(d) Redox reaction

4.Why did the cross mark disappear?
(a) The solution so formed has ability to absorb black colour hence black coloured cross is not seen.
(b) The resultant solution has high viscosity due to which it refracts the light to other angle.
(c) The resultant solution is dark coloured which does not allow the light to pass through it.
(d) The insoluble precipitate that is formed in the flask does not allow the light to pass through it to see the cross mark.