Preperiodic points for rational functions defined over a rational function field of characteristic zero

Let k be an algebraically closed field of characteristic zero. Let K be the rational function field K=k(t). Let ϕ be a nonisotrivial rational function in K(z). We prove a bound for the cardinality of the set of K-rational preperiodic points for ϕ in terms of the number of places of bad reduction and the degree d of ϕ.