根据短文内容，用方框中所给单词的适当形式填空，每词限用一次。                                                                                     

    Do you love animals? Would you like to keep animals as a pet? Nowadays, many people like to keep pets at home. The most popular____  are cats and dogs. They are friendly to people, and they are also ____  to take care of. Some people like parrots as pets. Pets lovers will____ lots of things for their pets, such as beds, clothes and even toothbrushes. Pets are very useful, too. The people who can’t hear need the dogs' help to answer the door. And the people who live____  should keep a pet if he wants to be healthier and ____ . If you have pets, don't forget to spend more time with them. They need love, too.

玉米叶肉细胞所含的核酸中，含有碱基A、C、T的核苷酸种类共有（　　）