human
Solve the following python programming problem given the following code:
@ mock.patch ('awscli.customizations.emr.sshutils.emrutils') 
def test_ssh_scp_key_file_format(self, emrutils) : 
    def which_side_effect(program) : 
        if ((program == 'ssh') or (program == 'scp')) : 
            return '/some/path' 
emrutils.which.side_effect = <MASK> 
    key_file1 = 'key.abc' 
    sshutils.validate_ssh_with_key_file (key_file1) 
    sshutils.validate_scp_with_key_file (key_file1) 
    key_file2 = 'key' 
    sshutils.validate_ssh_with_key_file (key_file2) 
    sshutils.validate_scp_with_key_file (key_file2) 
    
    Given the code above, what is a proper replacement for <MASK>? Choose among: key_file1, emrutils, self, key_file2
assistant
#Here is a proper replacement for <MASK>.
def which_side_effect(