notebook报错name 'SelectKBest' is not defined

下面是源码 前面没有报错

%matplotlib inline
import matplotlib.pyplot as plt
score = []
for i in range(350,200,-10):
      X_fschi = SelectKBest(chi2, k=i).fit_transform(X_fsvar, y)
      once = cross_val_score(RFC(n_estimators=10,random_state=0),X_fschi,y,cv=5).mean()
      score.append(once)
plt.plot(range(350,200,-10),score)
plt.show()