Why should one always use purest monomer in free radical polymerisation reaction?

Question

$\begin{array}{1 1}(a)\;\text{Impurities may slow down reaction rate}\\(b)\; \text{Impurities may accelerate the reaction rate}\\(c)\;\text{ Impurities may act as undesired catalyst}\\(d)\;\text{Desired product cannot be obtained}\end{array}$