Sometimes Children Will Correctly Use Irregular Words Such as Feet

Sometimes children will correctly use irregular words such as feet or went for a time and then begin making errors such as foots or goed.What do these errors indicate?

A) that the child is spending too much time hearing other children's errors
B) that the parents have not provided sufficient reinforcement and feedback for correctly using these words
C) that the child is highly motivated
D) that there has been an advance in the child's syntactical knowledge