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)
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