Unexpected behavior of scanf , asks two variable values at the same time

Question

enter image description hereWhen I run the program, first it asks to "write letter A" and then ask to "write letter B and write letter C" at the same time WHY?

#include <stdio.h>

int main()
{
  char a, b, c;

  printf("write letter A: ");
  scanf("%c",&a);
  printf("write letter B: ");
  scanf("%c",&b);
  printf("write letter C: ");
  scanf("%c",&c);

  return 0;
}