填入下面横线处与上下文衔接最恰当的一句是（ ）辣，我们都不陌生，很多人无辣不欢甚至吃辣上瘾，这是因为辣椒素等辣味物质刺激舌头、口腔的神经末梢时，会在大脑中形成类似灼烧的感觉，机体就反射性地出现心跳加速、唾液及汗液分泌增多等现象， ，内啡肽又促进多巴胺的分泌，多巴胺能在短时间内令人高度兴奋，带来“辣椒素快感”，慢慢地我们吃辣椒就上瘾了。

It was announced that only when the fire was under control ____to return to their homes. [ ]

There is no doubt ____ we will host the 2010 World Expo successfully

I don’t like watching documentaries because they are boring. — _______ .

She had almost failed the exam, but her sister, ________had done very well.[ ]