What precursor would accumulate in the following double muta

What precursor would accumulate in the following double mutants: c9, c10: c10, c14: Feeding experiment and biochemical pathway. From a mutagenesis screen, five mutants (named c9, c10, c14, c18, c21) were identified and they each were found to mutate a different gene. Feeding experiments were done by growing each mutant on minimal medium supplemented by four different known precursors of cytosine (A-D). The results are summarized in the table (+: growth; -: no growth): Show a simple linear biochemical pathway of the four precursors and the end product (cytosine). Indicate which step is blocked by each of the five mutations.

Solution

c18 c14 c9 c10 c21
---->D----->B---->A---->C----->Cytosine

So, the sequence of pathway shows that D is the first precursor and C is the immediate precursor to cytosine.