In which of the following ways of booking does one probably have to pay extra money

A．the development of babies' early forms of language
B．the difficulties of babies in learning to speak
C．babies' strong desire to communicate
D．babies' intention to communicate

A．no explanation is necessary for such a common case
B．no explanation has been made up to now
C．it's no easy job to provide a satisfying explanation
D．it's high time that an explanation was provided