执行下面程序后，画布上会出现几只海龟？

import turtle

t1=turtle.Turtle('turtle')

t2=turtle.Turtle('turtle')

t3=turtle.Turtle('turtle')

t4=turtle.Turtle('turtle')

t1.forward(50)

t2.forward(100)

t3.forward(150)

t4.forward(200)