C Program Output Behavior

ALTEN Belgium's embedded systems interviews often include a short C program and ask the candidate to reason through it line by line, since so much embedded work comes down to correctly predicting how integer types actually behave at the byte level, not just what the code seems to say.

#include <stdio.h>

int main(void) {
    unsigned char sensor_min = 20;
    unsigned char sensor_max = 15;
    unsigned char delta = sensor_max - sensor_min;

    printf("delta: %u
", delta);

    int readings[5] = {10, 20, 30, 40, 50};
    int i = 0;
    int sum = 0;

    while (i < 5) {
        sum += readings[i];
        i++;
    }

    short average = sum / 5;
    printf("average: %d
", average);

    unsigned int a = 3;
    int b = -1;

    if (a > b) {
        printf("a is greater
");
    } else {
        printf("b is greater
");
    }

    return 0;
}

Trace through this program and state exactly what it prints to the console, in order. Explain the reasoning behind each line, including any type conversions involved.