归因危险度（AR）
参考答案：又叫特异危险度、危险度差、超额危险度，是暴露组发病率与对照组发病率相差的绝对值。
Though Lei Feng ________ for fifty years, we are still learning from his spirit.
A．has died
B．died
C．has been dead
D．had died
焦炭的堆比重比烧结矿小。（）
