Four Cubes Outlines Puzzle - Solution

The Puzzle:

In the illustration four flat cube-like shapes are shown. Their patterns are drawn with bold black lines. Which of them can you draw without taking your pencil off the paper or going along the same line twice? Which of them can't be drawn in this way?

Author: Peter Grabarchuk
This Puzzle © 2008 Peter Grabarchuk. All Rights Reserved.
More from Peter Grabarchuk at PeterPuzzle.com

Our Solution:

Shapes A and D can be drawn without taking your pencil off the paper or going along the same line twice. Shapes B and C can't be drawn in this way.

Puzzle Author: Grabarchuk, Peter

See this puzzle without solution
Discuss this puzzle at the Math is Fun Forum