corrected Q13 - should add svg before .append("rect")

<!DOCTYPE html>
<head>
  <meta charset="utf-8">
  <script src="https://d3js.org/d3.v4.min.js"></script>
  <style>
    body { margin:0;position:fixed;top:0;right:0;bottom:0;left:0; }
  </style>
</head>

<body>
  <script>
   var svg = d3.select("body").append("svg")	
  	.attr("width", 960)
  	.attr("height", 960)
    //Select the svg and put some rectangles in it
  d3.select("svg").selectAll("rect") //DON'T ACTUALLY NEED THIS LINE
   		svg.append("rect") //modify the rectangles. 
  			.attr("y", 50)
  			.attr("x", 50)
      	.attr("width", 67.5) //this is the same for every rectangle
  			.attr("height", 15)
    		.attr("fill","#0055A4")
  	 svg.append("rect")
  			.attr("y",65)
  			.attr("x", 50) 
  			.attr("width", 67.5) //this is the same for every rectangle
  			.attr("height", 15)
    		.attr("fill","#FFFFFF")
 		svg.append("rect")
  			.attr("y",80)
  			.attr("x", 50)
  			.attr("width", 67.5) //this is the same for every recangle
  			.attr("height", 15)
    		.attr("fill","#EF4135")
  </script>
</body>

https://d3js.org/d3.v4.min.js