决定DSA信号强度的主要因素是（）
A.X线管电压
B.X线量
C.曝光时间
D.摄影部位
E.血管内碘浓度
参考答案：E
Alex changed his mind at the last minute，otherwise we ________ an agreement.
A．should have reached
B．could have reached
C．can reach
D．should reach
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