Analysis of Strain #2 Arg- mutants
Arg- Strain #2 grows with the addition of cit, but not orn: it also can grow with arg. Therefore, it can be inferred that orn precedes cit in the pathway. In the absence of a functional enzyme B, X cannot be converted to Y, but given Y, C can convert it to arg (which we already know to be the last product in the pathway), and growth results.
Alternatively, the expected phenotype of a mutation that affects enzyme B is that addition of any downstream product (in this case, Y or arg) will result in growth, but addition of any upstream product (in this case, X) will not. Since cit permits growth and orn does not, orn is the substrate of B, and cit is its product.