模型训练过程，正常，结果如下：
naiveBayesClassifier
Naive Bayes Classifier for Discrete Predictors
Call:
naiveBayes.default(x = X, y = Y, laplace = laplace)
A-priori probabilities:
Y
-1979 +1990 1980-1989
0.1153125 0.3043750 0.5803125
Conditional probabilities:
count
Y [,1] [,2]
-1979 76.46070 27.52961
+1990 71.94867 29.80863
1980-1989 75.12601 29.20054
avgRetweetCount
Y [,1] [,2]
-1979 0.3666456 1.2544417
+1990 0.2539568 0.6264289
1980-1989 0.3489661 1.3528286
avgReviewCount
Y [,1] [,2]
-1979 0.3447768 0.7766726
+1990 0.6835772 1.1494203
1980-1989 0.4447086 0.8397381
sourceCount
Y [,1] [,2]
-1979 15.98103 8.710137
+1990 14.77002 8.065539
1980-1989 15.46527 8.315700
但是在预测时，预测代码：prediction <- predict(naiveBayesClassifier, naiveBayesData[12, 2:5])
返回值：
prediction
factor(0)
Levels:
这是什么问题，求解答！！
