单项选择题
60钴治疗机照射野内有用射线空气比释动能率不对称性应小于()
A.5% B.10% C.15% D.20% E.60%
A.1mm B.1.5mm C.2mm D.2.5mm E.3mm
A.1mm B.2mm C.3mm D.0.5mm E.1.5mm