With .... End With

Bonjour à tous,
Comment rendre ce code moins long? Je suppose qu'on utilise With et End With, mais je ne sais comment cela fonctionne... Pouvez vous m'aider? Merci beaucoup

Range('a1').Activate
With ActiveCell
With .Font
.Bold = True
.Size = 10
End With
End With

Range('b1').Activate
With ActiveCell
With .Font
.Bold = True
.Size = 10
End With
End With
Range('c1').Activate
With ActiveCell
With .Font
.Bold = True
.Size = 10
End With
End With
Range('d1').Activate
With ActiveCell
With .Font
.Bold = True
.Size = 10
End With
End With