执行下列代码后，得到的结果是______。import tensorflow as tfx = tf.Variable([1., 2.])y = tf.Variable([3., 4.])with tf.GradientTape() as tape: f = tf.square(x) + 2*tf.square(y) + 1df_dx, df_dy = tape.gradient(f, [x, y])print("df_dx:", df_dx.numpy())print("df_dy:", df_dy.numpy())
选项：

A:df_dx: [15. 21.]df_dy: [12. 16.];
B:df_dx: [12. 16.]df_dy: [2. 4.];
C:df_dx: [2. 4.]df_dy: [12. 16.];
D:df_dx: [12. 16.]df_dy: [15. 21.]