下列抗菌药物中哪一种为窄谱抗菌药()
A．万古霉素
B．丁胺卡那霉素
C．阿齐霉素
D．磷霉素
E．环丙沙星
参考答案：A
Some old men _______ chess under the tree in the park now. [ ]
A. is playing
B. playing
C. are playing
D. eating
设
，当r≠0时有连续的二阶偏导数且满足
则u=u(r)满足的常微分方程是______.
，当r≠0时有连续的二阶偏导数且满足