在一定的总转化率下，单程转化率高，表明回炼比（）。
A、小
B、大
C、不变
D、不确定
参考答案：A
Not until he had left his friend _______ how important friendship was for him.
A．did he realize
B．he realized
C．had he realized
D．he had realized
回旋参数
