A：had only talked
B：am only talking
C：have just talked
D：was just talking
若x0是方程ax2+2x+c=0（a≠0）的一个根，设M=1﹣ac，N=（ax0+1）2，则M与N的大小关系正确的为（ ）
It's certainly hard work. But, a man who wishes to have a career has to make a great many sacrifices.
世界人口比较稠密的地区多位于（ ）
