求如图所示的交变电流的有效值，其中每个周期的后半周期的图象为半个周期的正弦曲线（

March 20th is World Water Day every year. According to a report from the United Nations, 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 earth. But now it has a serious problem.    小题1:   In most of the past 35 years, it didn't reach the sea.
The Nile in Egypt, running 6, 650 km, is the longest river in the world. 小题2:Now it is down to 2 billion m' or so.  Australia's Murray River reaches the sea only every other year.  One fifth of America's Rio Grande has no water, either.
____小题3:   
Mostly it's damming (筑坝) and global warming that make rivers dry.
____小题4:   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.
Global warming is making things worse. Warmer weather takes water away.  The glacier (冰川) are becoming smaller.   小题5:   Global warming has had certain effects on water sources (源头). Droughts (干旱) happening around the world make people pay more attention to this fact.

A．Dams hold water to irrigate (灌溉) fields and make power. B．The source of the Yellow River is drying up because of it. C．It used to carry 32 billion m' of water a year. D．How does it happen? E. Factories pour waste water into rivers directly. F. Its source on Qinghai-Tibet Plateau is drying up.