A live fish in a tank contains 100 Giardia parasites. After 20 minutes outside the tank, how many parasites are there?

The correct answer is determined by understanding the survival characteristics of Giardia parasites. Giardia is a protozoan that typically requires a host's environment to survive and reproduce effectively. When taken out of a water tank, many parasites would not be able to survive long-term due to the loss of their aquatic habitat, which is essential for their life cycle.

However, in the 20-minute timeframe mentioned, the grams of environmental stress may not have sufficiently impacted the number of parasites present in the fish. Since this time is relatively short, it's reasonable to assume that the initial number of Giardia parasites remains unchanged during that duration. Therefore, after 20 minutes, the number of parasites in the fish effectively remains at 100, as the environmental conditions haven't drastically decreased their numbers just yet. This understanding reinforces why the answer indicates that the quantity is still 100 parasites.