问题
选择题
21. He decided not to attend her birthday party________he had been invited to.
A．as if
B．even though
C．because of
D．Since
答案
答案：B
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21. He decided not to attend her birthday party________he had been invited to.
A．as if
B．even though
C．because of
D．Since
答案：B
的牙周膜、牙槽骨及唇侧牙龈受哪一神经支配（）。