阅读短文及文后A、B、C选项，选出可以填入1—3各题空白处的最佳选项。

According to a report from the United Nation, more than half of the world’s major rivers have serious water loss and are polluted.

The Yellow River, the world’s fifth longest river, watered some of the oldest farms on the Earth.【小题1】Its source (源头) on Qinghai-Xizang Plateau (青藏高原) is drying up. In most of the past 35 years, it didn’t reach the sea.

【小题2】

First, rivers are dry because of damming (筑坝). Dams provide fields with water and make power. Now about 45,000 of them lie across the world’s rivers. They hold back around 15% of the world’s fresh water from running to the sea.

【小题3】Warmer weather takes water away. The glacier (冰川) is becoming smaller. The source of the Yellow River is drying up because of it. Global warming has had certain effects on water sources.

A．How does it happen?

B．But now it has a serious problem.

C．Global warming is making things worse, too.