题目

.除去粗盐中可溶性杂质（SO42-、Mg2+、Ca2+等）需要下列实验操作：①过滤  ②加入适量的稀盐酸  ③加入过量的NaOH溶液  ④加入过量的BaCl2溶液  ⑤加入过量的Na2CO3溶液   ⑥蒸发，下列操作顺序错误的是：（    ） A. ③④⑤①②⑥               B. ④③⑤①②⑥ C. ④⑤③①②⑥               D. ⑤③④②①⑥() 8. —The problem is too hard. I don't know it out.—You can ask your teachers for help.   A. who to work   B. when to work   C. what to work   D. how to work