下列药，擅治阳明经头痛的是
A．羌活B．葛根C．白芷D．柴胡E．细辛
参考答案：C
是否存在非负整数a，b，使得|3a-2b|=41成立？若存在，求出a，b；若不存在，说明理由．
As it turned out to be a small house party, we （） so formally.
A．needn't dress up
B．did not need have dressed up
C．did not need dress up
D．needn't have dressed up
