执行下面程序后，以下哪个图形是正确的？（ ） 

import turtle
turtle.shape('square')
turtle.home()
turtle.dot()
turtle.stamp()
turtle.forward(100)
turtle.setheading(90)
turtle.stamp()
turtle.forward(100)
turtle.left(90)
turtle.stamp()
turtle.forward(100)
turtle.left(90)
turtle.stamp()
turtle.forward(100)