9/5 C: if-else if-else

#include <stdio.h>

int main(){
    int score;
    scanf("%d", &score);

    /*
        (X) 90 <= score <= 100
            score = 95:  90 <= 95 <= 100
                      =>    1     <= 100
                      => 1
            score = 89:  90 <= 89 <= 100
                      =>    0     <= 100
                      => 1
    */

    if( (score >= 90) && (score <= 100) ){
        printf("A+");
    }
    else if( score >= 85 && score <= 89 ){
        printf("A");
    }
    else if( score >= 80 && score <= 84 ){
        printf("A-");
    }
    // ........
    /*
        =:  score =  100 => 100
       ==:  score == 100 => 等於   : 1
                            不等於 : 0
    */
    else if( score == 0 ){
        printf("X");
    }


    return 0;
}