[多选题] 下列关于合同的成立与生效的相关描述正确的有()
A. 当事人采用合同书形式订立合同的,自双方当事人签字或盖章时合同成立 ;
B. 当事人采用信件、数据电文等形式订立合同的,可以在合同成立之前要求签订确认书,签订确认书时合同成立 ;
C.书面委托代理的授权委托书应当载明代理人的姓名或者名称,代理事项、权限和时间,并由委托人签名或者盖章 ;
D. 委托书授权不明的,被代理人可不向第三人承担民事责任

Every artist knows in his heart that he is saying something to the public. Not only does he want to say it well, but he wants it to be something which has not been said before. He hopes that the public will listen and understand -- he wants to teach them, and he wants them to learn from him.
What visual artists like painters want to teach is easy to make out but difficult to explain. Because painters translate their experiences into shapes and colors, not words. They seem to feel that a certain selection of shapes and colors, out of the countless billions possible, is exceptionally interesting for them and worth showing to us. Without their work we should never have noticed these particular shapes and colors, or have felt the delight which they brought to the artist.
Most artists take their shapes and colors from the world of nature and from human bodies in motion and rest; their choices indicate that these aspects of the world are worth looking at, that t
A. understand him and learn from him
B. notice only shapes and colors in his work
C. teach him something interesting
D. believe what he says in his work