Suppose that family F of stable maps given by maps $f:C \to S,\mu:C \to P^r$ and sections $\rho_i:S \to C$
Suppose that $\Sigma(F)$ be union of all one dimensional components of locus of nodes in fibers of $f$ and let $\pi_{F}:N(F) \to C$be normalization of $C$ along $\Sigma(F)$
My question is why we can choose some base change $\phi:S' \to S$ such that $\pi_{F'}^{-1}(\Sigma(F'))$ is disjoint union of sections of $N(F') \to S'?$
($F'$ is pullback family induced by $\phi$)