Which one is right about underfitting and overfitting?
选项：

A:Underfitting means poor accuracy both for training data and unseen samples.;
B:Overfitting means high accuracy for training data but poor accuracy for unseen samples.;
C:Underfitting implies the model is too simple that we need to increase the model complexity.;
D:Overfitting occurs too many branches that we need to decrease the model complexity.