若有以下定义,则对变量 student1 中 birthday 的错误赋值方式是 ( ) 。 struct student { int num; char name[20], sex; struct { int year, month, day; } birthday; } ; struct student student1; A. student1.birthday.year = 20 19 ; student1.birthday.month = 5; student1.birthday.day = 1; B. year = 20 19 ; month = 5; day = 1; C. birthday.year = 20 19 ; birthday.month = 5; birthday.day = 1; D. student1.year = 20 19 ; student1.month = 5; student1.day = 1;