对于强贯穿辐射，国际辐射防护委员会建议环境剂量当量中测算深度为（）
A.10mm
B.15mm
C.20mm
D.30mm
E.50mm
参考答案：A
随机区组设计方差分析中，必然有（）
His grandfather _______ for thirty years. [ ]
A. died
B. was head
C. has been dead
D. has died
