题目

Some problems ___ in the lecture by Mr.Smith are very popular in our school, such as the waste of electricity, water and food, etc... A. referring      B. referred to   C. referring to    D. were referred 答案：B 解析: 略设集合，若集合只有一个子集，则的取值范围是                                               （    ）A．B．C．D．