史料大致可以分为两类：一是直接的史料；二是间接的史料。凡是未经中间人修改或转写的是直接史料；凡是已经中间人修改或转写的是间接史料。下列属于直接史料的是（   ）

已知实数a，b，c，d满足a＋b＋c＋d＝3，a²+2b²+3c²+6d²=5 ，则a的最大值是(    )

历史的学习，需要由清晰的时序感，按发生时间先后顺序排列的是（    ）

①中共七届二中全会    ②渡江战役    ③三大战役    ④ 重庆谈判

选择最恰当的词，并用其正确形式填空，每词限用一次。

    It was Mother's Day, Nancy was 800 miles away from her parents, so she was a little ____ . In the morning she ____her mother to wish her a happy Mother's Day, and her mother told her about the beautiful flowers in the garden. Later that day, ____  she told her husband about the flowers, he said, "I know where we can find all that you want. Get our children Peter and Lily, and come on." So they went, ____  down the country roads.  
     There on a ____  , they saw a lot of beautiful purple flowers. Nancy ran ____    to enjoy the flowers. Carefully, she picked a few here and a few there. And on their way home, there was a big smile ____ her face.
     When they were passing a nursing home, she saw an old granny sitting in a chair. The granny had ____ children with her. They stopped the car and Nancy walked to the old woman, put the flowers in her hands, and smiled at her. The old granny ____  her again and again. She smiled happily, too.
     When Nancy ____ to her car, her children asked her, " Who is that old granny? Why did you give our flowers to her?”
" I do't know her.” Nancy said, "But it's Mother's Day, and she is alone. But I have both of you, and I still have my mother. Just think how much those flowers meant to her.