When about one of the biggest problems today, many experts say that an energy crisis is approaching and is thus threatening mankind's survival.
He insisted ______ alone in the mountain.
Xi’an was attacked by such a terrible smog _____ few residents had ever experienced before.
____ an important decision more on emotion than on reason, you will regret it sooner or later.
The way _____ was thought of by him was similar to the way ______ we carried it out.
You can not imagine what great difficulty we had _____ the home for the old.