以下代码实现将鼠标移到按钮上时按钮变红，鼠标移开时按钮变蓝，划线处的代码是？（ ）
from tkinter import *
root = Tk()
root.title( )
root.geometry('450x350')
btn1 = Button(root,text = '1')
btn1.place(x =200,y = 50,width = 40,height = 40)
def changebg(event):
# 鼠标移到按钮上按钮变红
event.widget['bg'] = 'red'
def changebg1(event):
# 鼠标离开按钮上按钮变蓝
event.widget['bg'] = 'blue'
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btn1.bind('<Leave>',changebg1)
root.mainloop()
btn1.bind()
btn1.bind('<Enter>',changebg)
btn1.bind('<Enter>',changebg1)
btn1.bind('<Button-1>',changebg1)