题目

In front of the house _____ which came here to be in search of several missing children.  A．several police cars were                           B．were several police cars C．was several police cars                             D．several police cars was 答案：B化肥中的氮肥很多是铵盐。已知氨气通入水中后，其溶液呈碱性。将硫酸铵和氢氧化钙混合后放入试管加热，用湿润的红色石蕊试纸放在试管口，湿润红色石蕊试纸会变蓝色，这是检验铵盐的方法。上述实验现象说明（ ）A. 铵盐和碱的反应是属于复分解反应类型B. 氨气比空气轻C. 能使红色石蕊试纸变蓝色就是氨气D. 铵盐能和碱反应放出氨气