问题
选择题
| Experts are trying to work out a plan to deal with the problems caused by global warming _____ it is too late. |
A. when B. if C. before D. until |
答案
答案：C
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| Experts are trying to work out a plan to deal with the problems caused by global warming _____ it is too late. |
A. when B. if C. before D. until |
答案：C
cm。棱镜材料的折射率为n=
。在此截面所在的平面内，一条光线以45o的入射角从AC边的中点M射入棱镜(如图所示)求：