Help Please: Income tax calulator with Functions and Robus validation

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

float tax(float t);
void clear (void); 
main()
{   
    do
    {
      float income, pc,hi, charity, t, taxincome, net;
      char a[0], b[0],c,choose;
      int nod,ch,flag=0;
      int person = 4000;
      
      printf("\n Choose one of the following options:");
      printf("\n 1. Tax calcultor:");
      printf("\n 0. Exit");
                 printf("\n Income tax calculator");
                 printf("\n =====================");
                 printf("\n Income for the current month <in thousand VND>:"); scanf("%f%c", &income,&c);
                 while (1)
                 {
                       printf("\n Pension contribution <5%%> y/n?"); scanf("%s", &a);
                       if (a[0]=='y' || a[0]=='Y' || a[0]=='n' || a[0]=='N')
                       break;
                       else
                       printf("\n You must press 'Y' or 'N' only, try again");
                 }
                 printf("\n");
                 while (1)
                 {
                       printf("\n Health insurance <1%%> y/n?"); scanf("%s", &b);
                       if (b[0]=='y' || b[0]=='Y' || b[0]=='n' || b[0]=='N')
                       break;
                       else
                       printf("\n You must press 'Y' or 'N' only, try again");
                 }
                 printf("\n Number of Dependants <18 years old:"); scanf("%d", &nod);
                 printf("\n Charity:"); scanf("%f", &charity);      
                 if (a[0] =='y' || a[0] == 'Y')
                 pc=income*0.05;
                 else 
                 pc=0.0;
                 if (b[0] == 'y' || b[0] == 'Y')
                 hi=income*0.01;
                 else
                 hi=0.0;
                 t = income-(4000.0 + pc + hi + 1600.0*nod + charity);
                 taxincome = tax(t);
                     printf("\n Gross Income: %.1f", income);
                     printf("\n Deduction");
                     printf("\n Personal allowance :    %.1d", person);
                     printf("\n Pension contribution:   %.1f", pc);  
                     printf("\n Health insurance:       %.1f", hi);
                     printf("\n Dependant:              %.1f", 1600.0*nod);
                     printf("\n Charity:                %.1f", charity);
                     printf("\n -----------------------------");
                     printf("\n Total:                  %.1f", t); 
                     printf("\n Taxable Income:         %.1f", taxincome);
                     printf("\n Net income:             %.1f", income-taxincome);
                     printf("\n =============================");
                     printf("\n Another run <y/n>?");
      }
      while (getch() == 'y' || getch() == 'Y');
      getch();
}
float tax(float t)
{
          float taxincome;
          if (t <= 5000)
             taxincome = t*0.05;
          else if (t <= 10000)
               taxincome = 5000*0.05 +(t-5000)*0.1;
          else if (t <=18000)
               taxincome = 5000*0.05 + 5000*0.1 + (t-10000)*0.15;
          else if (t <=32000)
               taxincome = 5000*0.05 + 5000*0.1 + 8000*0.15 + (t-18000)*0.2;
          else if (t <=52000)
               taxincome = 5000*0.05 + 5000*0.1 + 8000*0.15 + 14000*0.2 + (t-32000)*0.25;
          else if (t <= 80000)
               taxincome = 5000*0.05 + 5000*0.1 + 8000*0.15 + 14000*0.2 + 20000*0.25 + (t-52000)*0.3;
          else 
               taxincome =  5000*0.05 + 5000*0.1 + 8000*0.15 + 14000*0.2 + 20000*0.25 + 28000*0.3 + (t-80000)*0.35;
          return taxincome;
}