A:西瓜瓤
B:天竺葵叶片
C:花生子叶
D:苹果
副热带海区盐度高的主要原因是( )
大卫的成功不是偶然的,那是他艰辛努力的结果。
David's success didn't come ____ ____ . It was the result of his hard work.