我对以下哪一项应该用于标准化感到困惑：
方法一：拟合变换训练数据，只变换测试数据
from sklearn.preprocessing import StandardScaler sc = StandardScaler() X_train = sc.fit_transform(X_train) X_test = sc.transform (X_test)方法 2：拟合转换训练和测试数据
from sklearn.preprocessing import StandardScaler sc = StandardScaler() X_train = sc.fit_transform(X_train) # scaler_train=sc.fit(X_train) #X_train_sd=scaler_train.transform(X_train) X_test = sc.fit_transform (X_test) #scaler_test=sc.fit(X_test) #X_test_sd=scaler_train.transform(X_test)