连续输入两个a 会发现一个问题。我特想知道为什么会出现这样的问题？？

#include<stdio.h>
#include<stdlib.h>
float add (void);
float subtract (void);
float multiply (void);
float divide (void);
char choice (void);
int main()
{int ch;
  while ((ch=choice())!='q')
  {
      switch (ch)
      {
      case 'a': add();
          break;
      case 's':subtract();
          break;
      case 'm':multiply();
          break;
      case 'd':divide();
          break;
      default: 
          break;
      }
         
  }
   printf("bye\n");

     system("pause");
    return 0;
}
char choice (void)
{
char ch;
printf("Enter　the　operation　of　your　choice:\n");
printf("a.　add　　　　　　　　s.　subtract\n");
printf("m.　multiply　　　　 d.　divide\n");
printf("q.　quit\n");
ch=getchar();
while (getchar()!='\n')
{
    continue;
}

return ch;
} 
float add(void)
{float a,b;
float e;
char ch;
printf("Enter　first　number:");
while (scanf("%f",&a)!=1)
{   while ((ch=getchar())!='\n')
{
    putchar(ch);
}
printf("is not a number,");
    printf("you are wrong,enter again:");
      
}
printf("Enter　second　number:");
while (scanf("%f",&b)!=1)
{
    printf("you are wrong,enter again:");

    
}
e=a+b;
printf("%f + %f = %f\n",a,b,e);
return e;
}
float subtract (void)
{float a,b;
float e;
printf("Enter　first　number:");
while (scanf("%f",&a)!=1)
{
    printf("you are wrong,enter again:");
    
}
printf("Enter　second　number:");
while (scanf("%f",&b)!=1)
{
    printf("you are wrong,enter again:");
    
}
e=a-b;
printf("%f - %f = %f\n",a,b,e);
return e;
}
float multiply (void)
{float a,b;
float e;
printf("Enter　first　number:");
while (scanf("%f",&a)!=1)
{
    printf("you are wrong,enter again:");
    
}
printf("Enter　second　number:");
while (scanf("%f",&b)!=1)
{
    printf("you are wrong,enter again:");
    
}
e=a*b;
printf("%f * %f = %f\n",a,b,e);
return e;
}
float divide (void)
{
float a,b;
float e;
printf("Enter　first　number:");
while (scanf("%f",&a)!=1)
{
    printf("you are wrong,enter again:");
    
}
printf("Enter　second　number:");
while (scanf("%f",&b)!=1)
{
    printf("you are wrong,enter again:");
    
}
if (b==0)
{
 printf("Enter　a　number　other　than　0:");
 scanf("%f",&b);
}

e=a/b;
printf("%f / %f = %f\n",a,b,e);
return e;

}