单项选择题
There are altogether fifty students in our class, twenty ( )are boys.
A.of them
B.of those
C.of whom
D.of whose
A.
A.of
B.of
C.of
D.of
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【T13】A. ALL AF THAT B. AS IF IT WERE C. FRESH WATER SUPPLY D. IN THIS FIELD A. A LIMITED【T13】______ B. SO THAT IT CAN BE USED AGAIN JUST【T14】______ FROM A SPRING C. AND WE CANNOT EVEN USE【T15】______ D. EXPERIMENTS HAVE ALREADY BEEN DONE【T16】______ THE WORLD IS NOT ONLY HUNGRY, IT IS ALSO THIRSTY FOR WATER. IT SEEMS THERE IS PLENTY OF WATER, BUT ABOUT 97% OF THIS HUGE AMOUNT IS SEA-WATER, OR SALT WATER. MAN CAN ONLY DRINK AND USE THE OTHER 3% FRESH WATER.【T17】______ , BECAUSE SOME OF IT IS IN THE FORM. OF ICEBERGS AND GLACIERS. EVEN WORSE, SOME OF IT HAS BEEN POLLUTED. AT THE MOMENT, HIS SMALL AMOUNT OF FRESH WATER IS STILL ENOUGH FOR US. HOWEVER, OUR NEED FOR WATER IS INCREASING RAPIDLY. ONLY IF WE TAKE STEPS TO DEAL WITH THIS PROBLEM NOW, CAN WE AVOID A SEVERE WORLDWIDE WATER SHORTAGE LATER ON. ONE OF THE USEFUL STEPS WE CAN TAKE IS TO STOP UNLIMITED USE OF WATER.【T18】______ , HOWEVER, WOULD HAVE A BAD EFFECT ON AGRICULTURE AND INDUSTRY. IN ADDITION TO STOPPING WASTING OUR PRECIOUS WATER, ONE MORE USEFUL STEP WE SHOULD TAKE IS TO DEVELOP WAYS OF REUSING IT.【T19】______ , BUT ONLY ON A SMALL SCALE. TODAY, IN MOST LARGE CITIES, WATER IS USED ONLY ONCE AND IT EVENTUALLY RETURNS TO THE SEA OR RUNS INTO UNDERGROUND STORAGE TANKS. BUT IT IS POSSIBLE TO PIPE WATER THAT HAS BEEN USED TO A PURIFYING PLANT. THERE IT CAN BE FILTERED AND TREATED WITH CHEMICALS【T20】______