The doctor is skilled at treating heart trouble and never accepts any gift from his patients,so he has a very good_____.
A:expectation
B:reputation
C:contribution
D:civilization
半径为a的圆形区域内有均匀磁场,磁感应强度为B=0.2T,磁场方向垂直纸面向里,半径为b的金属圆环与磁场同心地放置,磁场与环面垂直,其中a=0.4m,b=0.6m.金属环上分别接有灯L1、L2,两灯的电阻均为R=2Ω,一金属棒MN与金属环接触良好,棒与环的电阻均忽略不计.
“山绕水绕,沃野千里”是对东北三省的山河大势最形象的描述,其中位于大、小兴安岭北部的河流是( )