The book was written in 1946, _____ the education system has witnessed great changes.

A．when

B．during which

C．since when

D．since then

据《建筑施工场界环境噪声排放标准》，建筑施工场界环境噪声排放限值为昼间（），夜间（）

A.65dB；55dB

B.75dB；60dB

C.70dB；50dB

D.70dB；55dB