input: Write a function to develop a decision tree classifier in Python to predict a fourth feature from three other features (A, B, and C) based on the provided input and output. The function should return the trained classifier.
  assert decision_tree_classifier() != None
output: def decision_tree_classifier():
      from sklearn.tree import DecisionTreeClassifier
      import numpy as np
      
      X = np.array([[1, 2, 3], [4, 5, 6], [7, 8, 9]])
      y = np.array([0, 1, 0])
      
      clf = DecisionTreeClassifier()
      clf.fit(X, y)
      
      return clf