下列生物中属于原核生物的一组是
①蓝藻 ②酵母菌 ③草履虫 ④念珠藻 ⑤水绵 ⑥青霉菌 ⑦葡萄球菌[ ]
A.①⑥⑦
B.①④⑥
C.①②③④⑤⑥⑦
D.①④⑦
答案:D
It was my first time I___________ Chairman Hu,and his speech made a deep impression on me. [ ]
A. saw
B. see
C. had seen
D. have seen
为什么要建立统一的突发事件信息系统?