题目

下列动物中，进行胎生、哺乳的是（ ） A . 海豚 B . 蜥蜴 C . 蝾螈 D . 啄木鸟 答案：AWith the help of high technology, more and more new substances ______ in the past years.  A. discovered                                                           B. have discovered       C. had been discovered                                            D. have been discovered