office etiquette

So you're at work and you pass a coworker in the hallway and you do the "Smile and say Hi" thing. Then you see them again less than an hour later (sometimes several times throughout the day). What do you do?

a.) Smile and say Hi ... again
b.) Say something witty about bumping into each other again (though these "witty" comments often provoke fake laughter followed by an awkward silence)
c.) Put your head down and smile, letting them know you see them but are not interested in further small talk
d.) Pretend you don't see them.

This seems to happen to me a lot. I usually just slightly smile upon repeated hallway appearances. It's when you see them from halfway down the hall that it's really awkward because you have an entire stretch of hallway to build up the appropriate response!

Hey, think I analyze things too much? :)