B细胞的阴性选择发生于（）
A.早期祖B细胞
B.晚期祖B细胞
C.不成熟B细胞
D.成熟B细胞
E.浆细胞
参考答案：C
粗准焦螺旋的正确使用方法是
①镜筒上升时，左眼向目镜内看；②镜筒下降时，左眼向目镜里看；③镜筒上升时，眼睛看着物镜与玻片间的距离；④镜筒下降时，眼睛看着物镜与玻片间的距离 [ ]
A．①④
B．②③
C．③④
D．①②
Open Shortest Path First(OSPF)is a (29) routing algorithm that (30) work done on the OSI IS-IS intradomain routing protocol. This routing, as compared to distance-vector routing, requires (31) processing power. The Dijkstra algorithm is used to calculate (32) OSPF routing table updates only take place when necessary, (33) at regular intervals.
32（）
A．routes
B．road
C．distance
D．cost
