Sentences given in each question, when properly sequenced, form a coherent paragraph. The first and last sentences are 1 and 6, and the four in between are labelled A,B,C and D. Choose the most logical order of these four sentences from among the four given choices to construct a coherent paragraph from sentences 1 to 6.
1. Security inks exploit the same principle that causes the vivid and constantly changing colours of a film of oil on water.
6. Since the distance the rays travel changes with the angle as you look at the surface, different colours look bright from different viewing angles.