北方有较多沙尘粒子，珠三角有较多有机气溶胶
参考答案：对
The book was written in 1946, ____ the education system has witnessed great changes.
A．when
B．during which
C．since then
D．since when
关于固定资产年折旧率的公式，表达不正确的是（）
A.
B.
C.
D.
