The hydrogen atom emits a photon when making a transition between energy levels ni → nf. Rank the transitions according to the wavelength of the emitted photon, largest to smallest. 

(a) 4 → 2; 

(b) 3→ 1; 

(c) 2 → 1; 

(d) 3 → 2; 

(e) 4 → 3; 

(f) 5 → 4.