Paper 2 · Contact process flow diagram; converting sulphuric acid to ammonium sulphate fertilizer

In the Contact process for manufacturing sulfuric acid, Fig.5's flow diagram shows sulfur and dry air entering box A; the process proceeds A -> B -> C, with oxygen also fed into C; C -> D, with water entering D and concentrated sulfuric acid leaving D; D -> E, with waste gases leaving E. In which box is sulfur dioxide oxidised to sulfur trioxide (the catalytic converter)?

Tap to enlarge

Derived from ZIMSEC Chemistry 4024/2 Paper 2 Theory, November 2015, Q9