The author thinks that the conclusions of any research about people's state of mind are ______

A．sign language was too artificial to be widely accepted
B．sign language was not extensively used even by deaf people
C．a language should be easy to use and understand
D．a language could only exist in the form of speech sounds

A．a new way to be taken at the learning of language
B．a challenge to traditional views on the nature of language
C．an approach to simplifying the grammatical structure of a language
D．an attempt to clarify misunderstanding about the origin of language