[chef] Which knife.rb?

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I'm attempting to bootstrap an EC2 instance and following the guide here:
http://wiki.opscode.com/display/chef/EC2+Bootstrap+Fast+Start+Guide#EC2BootstrapFastStartGuide-Step2:ConfigureAWSsettingsandtheknife.rbfile

One thing I'm a bit confused with is it says;

Add your AWS Cloud Credentials to the bottom of your knife.rb:
knife[:aws_access_key_id]     = "Your AWS Access Key"
knife[:aws_secret_access_key] = "Your AWS Secret Access Key"

Now, I've done a search of my file system and I can see a number of knife.rb
files;

./embedded/lib/ruby/gems/1.9.1/gems/chef-11.4.0/spec/support/shared/functional/knife.rb
./embedded/lib/ruby/gems/1.9.1/gems/chef-11.4.0/lib/chef/application/knife.rb
./embedded/lib/ruby/gems/1.9.1/gems/chef-11.4.0/lib/chef/chef_fs/knife.rb
./embedded/lib/ruby/gems/1.9.1/gems/chef-11.4.0/lib/chef/knife.rb

So my question is simply which one do I add the aws secret keys to?

Thanks in advance

John