请你帮帮我解一下这道题吧

#include <stdio.h>
int main()
{
  int score;
  scanf("%d", &score);
  while (score < 0 || score >100){
    printf("Error! Retry!\n");
    scanf("%d", &score);
  }
  if (score < 60)
    printf("Grade is E!\n");
  else if (score >= 60 && score < 70)
    printf("Grade is D!\n");
  else if (score >= 70 && score < 80)
    printf("Grade is C!\n");
  else if (score >=80 && score < 90)
    printf("Grade is B!\n");
  else
    printf("Grade is A!\n");
  return 0;
}