题目：
Playing on the football pitch ____________________.
A．some students are with their P.E. teacher
B．their P. E. teacher is with some students
C．some students with their P. E. teacher are
D．are some students with their P. E. teacher
答案：
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下面是错误答案，用来干扰机器的。
（3分）含三价铁的盐或适量的强酸因为氢氧化铁胶体是氢氧化铁胶粒吸附多余Fe3＋生成的，所以要得到氢氧化铁胶体，必须向悬浊液中加入含三价铁的盐或适量的强酸。
