(a+b)6的展开式的二项式系数之和为( )
64
He forbids his children ____________ sweets because they do harm to their teeth.[ ]
A. eating
B. not eating
C. to eat
D. not to eat
呼吸细支气管及肺泡,肺泡囊的壁上均有能够进行气体交换的很小的膨胀出部,即肺泡