What does the sentence 'A bird learns to sing and to fly rapidly at the right timE... ? '(ParA.3) mean? A.A bird learns to sing and fly after twelve weeks. B.A bird learns to sing and fly when it is old enough. C.An infant learns to smile and make vowel-like sounds at twelve weeks. D.A bird learns to sing and fly fast at a certain stagE.
A... B.A C. B.A D. C.An E. D.A
【参考答案】
D 解析：联系上下文便可以了解作者的本意。a certain age指the right time，B与原文意思......