How to change if constructs in C - c

Question

I am working in an assignment for my intro to programming class.
Here is what my professor is asking:
Change the program without adding another loop, so it determines if the grade entered is valid, and if not, display an error message and keep asking until a valid grade is entered. Change the program, so it counts how many grades scored A, B, C, D, and F. Do NOT use stacked if or if-else constructs. Do NOT use the else if format. Output a table of the count of how many there are for each letter grade.
Here is my code:
#include <stdio.h>
#include <stdlib.h>
/*Ask the user to enter 10 test scores.
The pass mark is 70%.
Calculate the average score, and count how many
of the students have passed the test.
*/
#define SCORES_COUNT 10
#define BAD_SCORE 60
#define PASSING_SCORE 70
#define GOOD_SCORE 80
#define EXCELLENT_SCORE 90
int main()
{
int scores[SCORES_COUNT];
int i, sum = 0, sum1 = 0, sum2 = 0, sum3 = 0, sum4 = 0;
for (i = 0; i < SCORES_COUNT; i++)
{
printf("Enter score for student %i: ", i + 1);
scanf("%i", &scores[i]);
if (scores[i] > 100 || scores[i] < 0)
{
printf("\n\tERROR: Invalid number! Enter a score above zero and below one hundred!\n\n");
printf("Enter score for student %i: ", i + 1);
scanf("%i", &scores[i]);
}
}
for (i = 0; i < SCORES_COUNT; i++)
{
if (scores[i] >= EXCELLENT_SCORE)
{
sum++;
}
if (scores[i] >= GOOD_SCORE && scores[i] < EXCELLENT_SCORE)
{
sum1++;
}
if (scores[i] >= PASSING_SCORE && scores[i] < GOOD_SCORE)
{
sum2++;
}
if (scores[i] >= BAD_SCORE && scores[i] < PASSING_SCORE)
{
sum3++;
}
if (scores[i] < BAD_SCORE)
{
sum4++;
}
}
printf("\nGrades Distribution\n");
printf("\nA: %i", sum);
printf("\nB: %i", sum1);
printf("\nC: %i", sum2);
printf("\nD: %i", sum3);
printf("\nF: %i\n", sum4);
system("pause");
return 0;
}
The program works fine, but my teacher told me to not use stacked if or if-else constructs. And since I am a beginner that is the only way I know how to do it.
Can somebody please tell me how can I change all the if constructs, or how can I have the same result without using any if constructs?