It is probably easier for teachers than for students to appreciate the reasons why learning English seems to become increasingly difficult once the basic structures and patterns of the language have been understood．

1965年9月17日，中国第一个用人工方法结合成了什么物质？这是世界上第几个人工合成的蛋白质？1998年4月15日，中国科学院上海生化所成功地运用什么方法重组了人工胰岛素？