属于壁胸膜形成的结构是E．

A. but
B. or
C. and
D. though

A. regulate
B. encounter
C. stimulate
D. challenge

A. challenged the status of public schools
B. helped promote the traditional culture
C. could destroy the unity of the country
D. placed heavy financial burdens on parents

A. unless
B. while
C. if
D. whether