题目

设是由满足下列性质的函数构成的集合：在定义域内存在， 使得成立．已知下列函数：①；②；③；④，其中属于集合的函数是    ▲   （写出所有满足要求的函数的序号）． 答案：．②④People in that country use spoons or chopsticks to eat. And it is impolite to use both of them ___.A. at that time B. at the same timeC. for the first time D. from time to time