题目

Now let us look at how we read. When we read a printed text, our eyes move across a page in short, jerky movement. We recognize words usually when our eyes are still when they fixate (注视). Each time they fixate, we see a group of words. This is known as the recognition span or the visual span. The length of time for which the eyes stop --- the duration of the fixation --- varies considerably from person to person. It also varies within any one person according to his purpose in reading and his familiarity with the text. Furthermore, it can be affected by such factors as lighting and下列实验不能达到目的是（ ）A．用AlCl3溶液和过量氨水制备Al（OH）3B．用NH4Cl和Ca（0H） 2固体混合加热制备NH3C．用NaOH溶液除去苯中的溴D．用足量铜粉除去FeCl2溶液中的FeCl3杂质