When babies babble "ba, ba, ba," they are demonstrating knowledge of a: ?

When babies are able to babble "ba ba ba" they are in the Canonical Babbling stage.  In this stage, babies are learning to recognize and order the sounds in order to form words.  This stage starts around 7-8 months of age and lasts until 10-15 months when the first words emerge. 

$))& $7& ))$$