目标的坚定是性格中最重要的力量源泉之一，青少年在确定人生目标时要注意 （   ）

光在____物质里传播的路线是直的；光在真空中的传播速度最大，数值是____米/秒．“立竿见影”的现象表明光在空气中是沿____传播的．

    A young man was in love with a beautiful girl. One day the girl said to him, “It is my birthday tomorrow.”“ Oh,” said the young man, “I'll send you roses, and one rose for each year of your life.”
     The same evening he went to the flower shop. As he knew the girl was twenty-two years old, he paid for twenty-two roses and asked the shop assistant to send them to the girl the next day. The assistant of the flower shop knew the young man very well as he often bought flowers in his shop before. When the young man left the shop, the assistant thought, “The young man is very kind to me. He often comes to buy my flowers. In return I'll send ten more roses.”
    He did so. The next morning thirty-two roses were sent to the girl. When the young man came to see her, she didn't want to speak to him. And he never knew why she was so angry with him.

美国作家维尔斯第一次来苏联的时候，称列宁是“克里姆林宫的幻想家”，表示出对社会主义的怀疑；1934年当他第二次来到苏联的时候，他对斯大林说：“现在资本主义应该向你们学习，理解社会主义精神。”他对社会主义认识的变化，基于（    ）