The Biofilm That Forms in Your Shower ("Shower Curtain Scum")

The biofilm that forms in your shower ("shower curtain scum") can tolerate exposure to hot water when you shower and is resistant to desiccation between showers.The microbes in the biofilm are not endospores, so what is the best explanation for their survival under these conditions?

A) Bioflms are composed of diverse microbes, and within each biofilm, there are certain to be endospore-formers and thermophiles.
B) The shower hasn't been thoroughly cleaned in a while, and when I really scrub it, the microbes will all be killed.
C) Although some microbes will be killed, those embedded deeply within the biofilm are protected from environmental challenges.
D) Dead skin cells continuously replenish the biofilm, and organic soaps provide a continuous carbon source.