.body {
  margin: 0;
  overflow: hidden;
  height: 100%
}

#A{
  margin-left:5,
  position: relative,
  display: block;
  float: left,
  width: 100%,
  /* height: 10em; */
  border: 1px solid black;
}

#B {
  marigin: 0,
  position: relative,
  display: block;
  float: left,
  width: 100%,
  /* height: 3em; */
  border: 1px solid black;
}