Monte Carlo integration of the Gaussian function f(x) = exp(-x^2/2) in C incorrect output

I'm writing a short program to approximate the definite integral of the gaussian function f(x) = exp(-x^2/2), and my codes are as follows:

#include <stdio.h>
#include <stdlib.h>
#include <math.h>

double gaussian(double x) {
    return exp((-pow(x,2))/2);
}

int main(void) {
    srand(0);
    double valIntegral, yReal = 0, xRand, yRand, yBound;
    int xMin, xMax, numTrials, countY = 0;

    do {
        printf("Please enter the number of trials (n): ");
        scanf("%d", &numTrials);
        if (numTrials < 1) {
            printf("Exiting.\n");
            return 0;
        }  
        printf("Enter the interval of integration (a b): ");
        scanf("%d %d", &xMin, &xMax);      
        while (xMin > xMax) { //keeps looping until a valid interval is entered
            printf("Invalid interval!\n");
            printf("Enter the interval of integration (a b): ");
            scanf("%d %d", &xMin, &xMax);
        }
        //check real y upper bound
        if (gaussian((double)xMax) > gaussian((double)xMin))
            yBound = gaussian((double)xMax);
        else 
            yBound = gaussian((double)xMin);
        for (int i = 0; i < numTrials; i++) {
            xRand = (rand()% ((xMax-xMin)*1000 + 1))/1000.00 + xMin; //generate random x value between xMin and xMax to 3 decimal places             
            yRand = (rand()% (int)(yBound*1000 + 1))/1000.00; //generate random y value between 0 and yBound to 3 decimal places
            yReal = gaussian(xRand);
            if (yRand < yReal) 
                countY++;
        }
        valIntegral = (xMax-xMin)*((double)countY/numTrials);
        printf("Integral of exp(-x^2/2) on [%.3lf, %.3lf] with n = %d trials is: %.3lf\n\n", (double)xMin, (double)xMax, numTrials, valIntegral);

        countY = 0; //reset countY to 0 for the next run
    } while (numTrials >= 1);

    return 0;
}

However, the outputs from my code doesn't match the solutions. I tried to debug and print out all xRand, yRand and yReal values for 100 trials (and checked yReal value with particular xRand values with Matlab, in case I had any typos), and those values didn't seem to be out of range in any way... I don't know where my mistake is.

The correct output for # of trials = 100 on [0, 1] is 0.810, and mine is 0.880; correct output for # of trials = 50 on [-1, 0] is 0.900, and mine was 0.940. Can anyone find where I did wrong? Thanks a lot.

Another question is, I can't find a reference to the use of following code:

double randomNumber = rand() / (double) RAND MAX;

but it was provided by the instructor and he said it would generate a random number from 0 to 1. Why did he use '/' instead of '%' after "rand()"?