The following auxotrophic mutant strains were isolated from wild type, haploid yeast and tested for growth on four precursors, A-D, known to be involved in the biosynthesis of thymine. Supplement added to minimal medium A B C D Thymine 9 10 14 18 21 Show a simple linear biosynthetic pathway of the four precursors and the end product, thymine. Indicate which step is blocked by each of the five mutations.