Why does it display "no such option" when I have not selected an option yet? Why / what are the errors in my code?

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


int main(){

  //declare variables
  int max, min, amount, num; 
  int sum = 0; 
  char opt;
  float mean;

  //prompt user to enter the minimum possible value 
  printf("Enter the minimum random number to be generated: "); 
  scanf("%d",&min);

  //prompt user to enter the maximum possibe value
  printf("Enter the maximum random number to be generated: ");
  scanf("%d",&max);
    
  //promt user to enter the number of random numbers
  printf("Enter the number of random numbers: ");
  scanf("%d",&amount);
  
  //time function to generate random numbers
  srand(time(NULL));
  
  printf("----------\n");

  //prints table with the serial number and the value of the generated random number
  for(int i=001;i<=amount;i++){

    num = (rand() % (max - min + 1)) + min; //generates random numbers
      
    if(i<=9){ //prints the first number to the ninth number
      printf("|00");   
      printf("%d | %d|\n",i,num);
    }
    
    else{ //prints numbers after the ninth number
      printf("|0");
      printf("%d | %d|\n",i,num); 
    }
  }
    
  printf("----------");

  //performs operations multiple times
  do{
    //displays options
    printf("\nSelect the option\n");
    printf("a - mean of all random numbers\n");
    printf("b - sum of all random numbers\n");
    printf("c - Exit\n");
    scanf("%c",&opt);
      
    //switch case for multiple choices
    switch(opt){
            
      case 'a': 
        for(int i=0;i<amount;i++){
          sum += num;
        }
        mean=sum/amount;
        printf("Mean is %f\n", mean);
        break;
            
      case 'b': 
        for(int i=0;i<amount;i++){
          sum += num;
        }
        printf("Sum is %d\n", sum);
        break;
          
      case 'c': 
        printf("Thank you!\n");
        break;
          
      default:
        printf("No such option\n");
    }
        
  }while(opt);