After the Aswan High Dam was built on the Nile River, the rate of parasitic blood-fluke infection doubled in the human population near the dam. As a result of building the dam, the flow of the Nile changed. This changed the habitat, which resulted in an increase in its population of a certain aquatic snail. The snails, which were infected, released larvae of the fluke. These larvae then infected humans.' This situation best illustrates that:
A. the influence of humans on a natural system is always negative in the long term
B. the influence of humans on a natural system can have unpredictable negative impacts
C. human alteration of an ecosystem does not need to be studied to avoid ecological disaster
D. human alteration of an ecosystem will cause pollution and loss of finite resources