Macro to selectively copy cells from excel.

Hi,
I am a novice in excel macro programming, and new to this forum. I need a help in developing a Macro - VB script for the following scenario.

I have 3 columns, Column A, Column B and Column C.
I want macro to check contents of Column B and wherever there is a blank, macro should copy correspoding content in Column A to Column C. Column C shouldn't have any blanks.

Ex:
Column A Column B Column C
abc 123 efg
cde 234 ijk
efg
ghi 456
ijk
klm 678

Any help will be much appreciated. Thanks in advance.