$1.$ Selectivity in parallel reactions is defined as the ratio of$\_\_\_$

a$)$ Moles of desired product formed to the moles of undesired material formed

b$)$ Moles of undesired product formed to the moles of desired material formed

c$)$ Moles of product formed to the moles of reactant consumed

d$)$ Moles of reactant consumed to the moles of product formed