human
Solve the following python programming problem given the following code:
def HandleClockDriftToken(metadata_watcher, on_change) : 
    'Watches for and responds to drift-token changes.

  Args:
    metadata_watcher: a MetadataWatcher object.
    on_change: a callable to call for any change.
  ' 
    clock_drift_token_key = 'instance/virtual-clock/drift-token' 
    def Handler(event) : 
        on_change (event) 
metadata_watcher.WatchMetadataForever (<MASK>, Handler, initial_value = '') 
    
    Given the code above, what is a proper replacement for <MASK>? Choose among: on_change, clock_drift_token_key, metadata_watcher, Handler
assistant
#Here is a proper replacement for <MASK>.
clock_drift_token_key