已知a∈R，则a=0是函数y=x2+ax+1为偶函数的（ ） A．充要条件 B．
题型：选择题
问题：
已知a∈R，则a=0是函数y=x2+ax+1为偶函数的（ ）
A．充要条件
B．充分不必要条件
C．必要不充分条件
D．既不充分也不必要条件
题型：选择题
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| When I was in the third grade, I wanted to be just like Mrs. Abshere. She always had a 1 for her students. She made us feel both special and wanted. I _2 felt that Mrs. Abshere cared about me. Her patience and kindness helped me 3 many hard times. One day, after I got home, the police suddenly came and arrested (逮捕) my parents. I was 4 to death! I held on to my brother and sister as we were 5 a children's home. As my parents waited for their 6 , my brother, sister and I waited in the children's home. Soon days turned into weeks. When I was told that they would be in prison for six months, I 7 on my bed and cried for hours. I was mad at the world and no longer saw any future for myself. Just at this very moment one of the girls came running up to me. "Hey, there's a lady here to  see you. I think it's your mom." _8_ itbe true? When I got to the end of the hallway, there stood Mrs. Abshere! She held her arms out to me. "Honey, I know 9 is better than your mom, but I love you, too!" I hugged her and 10 thanked God for bringing this angel thirty miles out of her way to see me. As I 11 Mrs. Abshere around the grounds, I began to feel 12 again. I was so excited to show her off to all my new friends. Two weeks later, I said good-bye to all my friends and moved across the state to live in another city with my grandparents. Sixteen years later, I 13 Mrs. Abshere, so that I could tell her how grateful (感激的) I was. When I found her, we had the most wonderful conversation. I felt so much 14 telling her that I was only a year away from becoming a teacher myself. "Oh! That's great. You can teach students in my room!" 15 , perhaps I will be returning to Mrs. Abshere's classroom soon, only this time as a peer (同行) and a very grateful friend. | ||||
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的三棱镜．欲使光线从棱镜的侧面AB进入，再直接从侧面AC射出，求入射角θ的取值范围为 。