几乎不能通过乳腺进入乳汁的维生素是（）
A．钙
B．铁
C．维生素A
D．维生素D
E．维生素B1
参考答案：D
Mary _____ go to school by bike, but now she usually takes a school bus to school. [ ]
A. used to
B. had to
C. was afraid to
D. preferred to
OLT PON口接收光灵敏度为（）dBm。
A.-22
B.-24
C.-25
D.-30
