Bathroom conundrum

When one is in the privacy of one's bathroom - and it doesn't necessarily have to be one's bathroom, any bathroom is appropriate - and one is going about one's business, a dilemma is in the making.
The dilemma that I refer to here is the situation in which an external force in the form of a knock is applied to the outside of the door. The door being, of course, the entry to the bathroom that one is currently occupying.
Several solutions avail themselves at this point. Does one cry out, "I'm in here" and endure the possible questions that might ensue? Who exactly is 'I' and what is the initiator of the knock going to do about it? And indeed, one most certainly (depending on where this bathroom is located; public dormitory as opposed to private home) will have ensured that admission is impeded via door-lockage, and as such, utterances akin to 'I'm in here', are redundant to say the least...
One might also cry out, "Occupied!". The ostentation and pageantry associated with such a manner of response need not be discussed.
So what, really, must one say in reaction to this happening?
"Yes?", "What do you want?" or even "Who's there?"
Perhaps one's best option is to remain silent, for even the most brainless of knock-instigators will understand the meaning of a locked door.
Still, the conundrum of the bathroom remains irresolvable. It lurks behind the shower curtain, underneath the bathroom sink, in the medicine cabinet, and not even the most socially cognizant individual can figure out its mysteries.