According to the author, three design types of previous researches are ______.

A．parents can never hope to teach their children new sounds
B．children no longer imitate people after they being to speak
C．even after they have learnt to speak children still enjoy imitating
D．children who are good at imitating learn new words more quickly

A．may have problems with their hearing
B．usually pay close attention to what they hear
C．often take a long time in learning to listen properly
D．probably do not hear enough language spoken around them