执行下面代码段，对其输出结果描述正确的是_______。import tensorflow as tfmnist = tf.keras.datasets.mnist(train_x, train_y), (test_x, test_y) = mnist.load_data()print("training set:", train_x.shape)print("testing set:", len(test_x)) A: training set:(60000,) testing set:10000 B: training set: (60000, 28, 28) testing set: 10000 C: training set:(10000, 28, 28) testing set: 60000 D: training set:(10000,) testing set: 60000