Can you help me with this Pathways for the synthesis of amin

Can you help me with this?

Pathways for the synthesis of amino acids in bacteria were worked out in part by cross-feeding experiments using mutant bacteria that are defective for individual steps in the pathway – one such experiment is shown on the right for three mutants with defective genes for enzymes in the tryptophan synthesis pathway. Each mutant is streaked on a Petri dish and allowed to grow briefly in the presence of a very small amount of tryptophan, producing three pale streaks. Heavier growth is observed where some streaks are close to other streaks, indicating that one mutant can cross-feed (supply an intermediate by diffusion through the media) to the other one. From the pattern of crossfeeding, deduce the order of the steps controlled by the products of the TrpB, TrpD and TrpE genes. Explain your reasoning.

Solution

cross-feeding experiments

A.