6. 解析:选D。考查词义辨析。optimistic乐观的;concerned关心的;mysterious神秘的;sceptical 持怀疑态度的。句意为：他们承诺说将在一个月内完成这个计划，但我对此表示怀疑。

5. 解析:选B。考查非谓语动词。句意为:一位社会学家在会上作了报告，阐明了留守儿童已经引起了公众的广泛关注。throwing light on作伴随状语；that引导同位语从句。故选B项。

4. 解析:选B。从后面的进一步说明“他是一个守信用的人”可以推断出“他肯定现在已经动身了”。对过去的肯定推测应该用must have done。could have done表示说话人对过去已发生的情况提出委婉的批评，意为“本来可以……; 其实当时/那时可以……的”。

3. 解析:选B。考查固定搭配。attack ...for因……而抨击；on education关于教育。句意为：这篇文章因为政府关于教育的政策而抨击了政府。

2. 解析:选D。考查动词短语辨析。stick to 坚持；devote to 致力于……；react to 对……反应；adapt to 适应……。根据句意:为了与时代同步，我们必须适应变化快速的社会。故选D。

5. He is always coming straight to the point when having a meeting, which we all like.

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4. It was unfair that you were sceptical about his feelings to you.

3. The soldiers were trained to be able to adapt to any difficult conditions.

2. It might have rained last night because the ground appears to be wet.

5. attacking