煤的灰分对爆炸性的影响还与挥发分含量有关，挥发分小于（）的煤尘，灰分的影响比较显著。
A.8%
B.10%
C.15%
D.20%
参考答案：C
1868年埃里奥特在哈佛大学实行了（）制。
The service life of distress signals must be not more than（）.
A.forty two months from the date of manufacture
B.thirty six months from the date of the last inspection
C.twenty four months from the date of approval
D.twelve months from the date of purchase
