如下图示,从图中能获得最有效的历史信息是( )
A:九一八事变
B:西安事变
C:七七事变
D:八一三事变
-- Li Ming said he would come here, _______ he hasn't arrived.
-- Don't worry. He is on the way. ______, there is much traffic, so it will take more time.
若下列三个二元一次方程:3x+y=5,x-3y=5,y=ax-9有公共解,那么a的值应是( )
读“聚落分布示意图”,完成下列要求: