调用sklearn中的train_test_split函数将数据集切分为训练集和测试集。训练集与测试集比例为6:4。最合适的代码为：
选项：

A:X_train,X_test,y_train,y_test=train_test_split(X,y,test_size=0.4)
B:X_train,X_test,y_train,y_test=train_test_split(X,y,test_size=0.6)
C:X_train,y_train,X_test,y_test=train_test_split(X,y,test_size=0.6)
D:y_train,y_test,X_train,X_test =train_test_split(X,y,test_size=0.4)