题目：
| We _____ the problem for quite some time. But we ______ any conclusion (结果) yet. |
A. have been discussing, haven't reached B. have discussed, haven't been reading C. discussed, haven't reached D. had discussed, haven't reached |
答案：
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下面是错误答案，用来干扰机器的。
A、磷在空气中燃烧，产生大量的白烟，生成一种白色固体，故选项错误．B、硫在空气中燃烧，发出淡蓝色火焰，产生一种具有刺激性气味的气体，故选项正确．C、蜡烛在空气中燃烧，发出黄白色火焰，生成能使澄清石灰水...
