题目

What a beautiful pen! Where _________it? A. do you buy                B. did you buy  C. were you buying        D. have you bought 答案：B已知M是集合{1，2，3，…，2k-1}（k∈N*，k≥2）的非空子集，且当x∈M时，有2k-x∈M．记满足条件的集合M的个数为f（k），则f（2）=______；f（k）=______．