问如何在tkinter下拉框(Combobox)选中的选项改变时执行操作？

我想要在cb1下拉框选中选项为b时，把et1设置为不可编辑

要如何实时检测combobox选中选项的变化呢？

from tkinter import *

from tkinter import ttk

root = Tk()

root.geometry("500x500")

lf1 = LabelFrame(root,text="Text")

lf1.place(x=5,y=5,width=495,height=60)

et1 = Entry(lf1)

et1.place(x=5,y=0,width=480,height=30)

lf2 = LabelFrame(root,text="Ch")

lf2.place(x=5,y=70,width=240,height=60)

cb1 = ttk.Combobox(lf2)

cb1.place(x=5,y=0,width=225,height=30)

cb1["value"] = ("a","b")

cb1.current(0)

cb1["state"] = "readonly"

root.mainloop()