One data analyst needs to build a decision tree model for a classification problem. She created a decision tree model, called DT$,$ by using DecisionTreeClassifier$()$ and its default parameters. $($see below$)$

DT $=$ DecisionTreeClassifier$()$

DT$.$fit$($train$\_$X$,$ train$\_$y$)$

What$$s likely to be the problem with DT$?$ Why?