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)