When exposed to ultraviolet light, some molecules show fluorescence. If the structure or shape of these molecules changes, the fluorescence intensity can also change. The enzyme tryptophan synthase shows bluish-green fluorescence because of a phosphate group linked with the active site. The enzyme catalyses the reaction shown. $\text{serine} + \text{indole} \rightarrow \text{tryptophan}$ When serine is added to the enzyme, the fluorescence intensity increases, but if indole is then added as well, the bluish-green fluorescence falls in intensity. What conclusion may be drawn from these observations?
- A1, 2 and 3
- B1 and 2 only
- C1 and 3 only
- D2 and 3 only